I originally posted this as feedback/suggestion however I was informed that this was a bug and told to write up a bug report for it. I will post the link to the feedback article and just do a TLDR of the issue.
Feedback Link: Affixing bad feels - #5 by Platinum
TLDR: Added an affix (+dex) to a lvl 12 quiver, got a critical success which upgraded an affix already on the item (+chance to slow). This upgrade caused the quiver to now require level 15 to use. My character is only level 13.
Yup, I’ve had this before though it’s pretty rare as you need to be very close in level to the item such that the crit pushes the item level requirement up over your character level.
Ditto… had the same problem just after they implemented this feature… Cannot say I have had it since the latest patch dropped tho but then I have not started a new toon lately either… Come to think of it, I dont think I recall when last I had a crit success… 
Thanks for the report!
I’ll close this as I’ve since moved the original thread here.