This seems to be a persistent bug, Turn-Based Mode turns itself off:
https://www.reddit.com/r/BaldursGate3/comments/15ss7qz/please_stop_taking_me_out_of_turn_based_mode/To reproduce:
1. Enter Turn-Based Mode to buff pre-combat with party close together, not separated, not un-chained from each other, nobody sneaking.
2. Initiate combat with one character,
3. Turn-Based Mode auto-toggles off for all other party members,
4. Poof, pre-combat buffs all run out in the first turn of combat, even though you never touched the Turn-Based Mode button after you turned it on the first time.
If the player switches Turn-Based Mode on for all characters, I can think of no reason for the game to automatically switch it off, especially when all PCs remain chained, closely grouped, and nobody's sneaking.
Am I missing something here?