Hi Branca -
This can sometimes happen when there are recent updates to things like your project calendar—such as changes to working hours or holidays—or if there are adjustments to task dependencies or constraints. Even for tasks set as “fixed duration,” these types of changes can cause the system to automatically recalculate the task duration.
If you’ve updated the calendar, tweaked resource assignments, or modified dependencies, those could be triggering what you’re seeing. It’s also worth checking your project templates, since misconfigurations there can sometimes lead to similar behavior.