Hi @Einecolabitte , good suggestions already. The one cause not yet mentioned that's worth checking, the parent task's progress and duration settings. If the parent task or a subtask has a manual Percent Complete override, or a Duration Type that isn't set to calculate from children, the parent won't roll up to 100% even when all subtasks are done. Parent percent complete is also weighted by subtask duration, so an unusual duration value on one subtask can throw the rollup off.
Between that, checking for a pending approval or open issue on the completed tasks, and recalculating the project timeline, one of those should resolve it. If everything looks correct and it still sticks, it may be a one-off glitch on that project and worth a support case so Adobe can check the backend state.