I noticed this series of events the other day, and was able to re-produce the effect easily. Just wanted to share in case this was not intended functionality.
Log file should not be required as this is easy to reproduce.
PlayerA: Has no timelines/monoliths unlocked (playerA wants “Reign of Dragons” lets say)
PlayerB: Has the timeline unlocked that playerA wants
PlayerA and PlayerB crate a party
PlayerB teleports to Monolith Island that PlayerA wants
PlayerA teleports to PlayerB
(PlayerA can note here that they can not interact with the Echo Device “This timeline is locked”
PlayerB runs one Echo(complete objective) with PlayerA
PlayerA survives the Echo and portals to reward room
PlayerA can now open Echos from their own Echo Device in the previously locked timeline
PlayerA can now gain Stability on previously locked Timeline
PlayerA with full stability can also run the boss and unlock the next timeline effectively skipping as many timelines as PlayerA would like
New timeline for PlayerA will have no bridges to or from the new island (before filling stability and running the relevant echo boss).
Final note: this gets even stranger if PlayerA wanted to unlock one of the final three islands like The Age of Winter.