That is good info for us to bring to a developer. It means it is triggered by something else everyone is doing that is locking up the portal.
Anything you can tell us that could narrow it down what could cause it to break would be helpful. We have tried numerous different interactions but haven’t triggered portals to break for us yet internally.
What vendor was this?
I don’t can call reproduction.
But I experience it after a full inventory when going to the vendor after the mono for selling stuff and/or doing forge. Usually shattering or remove affixes.
When entering the next Mono the portal is bugging.
The vendor in the monolith timeline
For me it started when doing a rift beast mono chain in a non-corruption timeline on my new non-season created character. I tried doing some other monos including monos and quest monos but all had bugged portals. Only restarting the game solved it. Even when I press M and teleport to the mono hub it doesnt fix it
The bug happened to me on my first monolith after finishing Heorot questline, so that was before I got to the vendor in the monolith timeline.
Exactly what I have experienced!
So i was trying to repeat this bug, but sadly i cant catch the reason
I got it 3 times during today (before evening)
Then i tried to pay attention on my actions
About 2,5 hours all was fine - i did many echoes in one monolith without problem (simple echoes: not weaver, not boss, not cemetries, not chains)
And then i got bug, but unfortunately lost my attention and dont really understand why it happened
All i remember: it was cache near my start place (i did open it), then i directly went to echo objective, did it, then nearby was nemesis, i killed it, then made portal and got bug
I explored about 40% of map, did not see mage, rift and omen
I was trying to do something similar to catch bug again, but couldnt
So still dont know the reason, probably i did something else but didnt notice it…
This happened to me by entering my first cemetary and defeating the boss, I enchanted my idol twice and then went to portal out and the portal disappeared.
Just ran around 20-25 Echoes, including normals, basics, Tombs, Woven echoes. The portal bug happened just now, in The Forgotten Keep, the echo where you need to slay The Baroness.
Prior to the update, the bug, literally, has been happening on the first echo compeltion. Could not play the game at all.
Also, I have not been able to find any characteristics of the bug, it just happened.
Hope it gets fixed soon, for I really wanna play the game, but was unable to since season realease 
The portal in the monoliths worked before this new season release. Something with the new mechanics must be causing the portal the fail. MTX portals worked fine in the prior season. As someone else stated earlier, the new mechanics with the rifts/fractures/nemesis is highly suspect as interrupting what worked fine in prior seasons.
I play both on my windows PC and steam deck. It never happens when playing on the PC, but it happens every time on the steam deck within 5ish echoes. Same character, both with controller, and same play style
Whatever the issue for me it doesn’t seem to be related to play style, interactions, character or time but something specific to the install on my deck.
After reading this section of comments, I stopped going to the vendor inside the monolith. For next 4 echoes, it worked, no problem. Then, I forgot and accidentally visited the vendor and on the next echoe I had the no-exit-portal problem.
So, maybe vendor is the problem here.
Happened to me while in an echo chain. I used the echo chain portal to go through a couple of echoes, and then the portal out from within the echo was bugged. I am guessing it does not know where to return me to.
Further, I used the map waypoint to go back to the timeline waypoint. Then I went from there to the next echo in the chain. I died in that echo and when I respawned, I was still in the echo! I could die and respawn within the echo over and over. Again, I think it did not know where to send my character back to.
SteamOS Legion Go, full offline.
Edit to add: It was an Omen chain
Edit 2: Also, the “Open portal” button was not appearing
It might be related to a specific timeline (iirc I was getting it most often in The Black Sun timeline). It may also have something to do with the distance from the center island of the timeline - I only started seeing the bug once I was 6-8 echoes out from the center.
I got bug in other timeline (actually others) and not so far from center
From what I discovered so far the portal bugs out after interacting with the vendor from The End of Time town and with the vendor from finished echo waiting room .
Hope this helps !
Steam deck, offline mode.
Just reached endgame and the portal opens but then vanishes when I press A.
It hasn’t worked once for me after about 6 tries. Each time I restart the game, visit the vendor at the end of time, buy runes of shattering, open forge in the wheel, break down my inventory or add/change affixes, then go to the stash and place my idols/uniques, then try the next map.
The previous map shows as complete and the timeline stability is correct.
Only other thing that’s different from past runs is I have a bunch of corrupted gear and idols.
I’m playing offline mode on Steam Deck.
Edit: I’m confirming the merchant in the monolith echo area. Once selling off items, the next echo I do I can’t portal out of.
I’m playing in offline mode and I am having the same problem ;(.
