This is by design if the resource is over allocated for the duration of the task, as even fixed duration tasks duration will increase when exported to MSP to accommodate for the over allocation. But unlike other task types that will just adjust the task duration without warning, you will be warned by MSP with Fixed Duration tasks that the duration is increasing. The error is only generated for one task (the last task in the WBS with the issue), so if the issue is corrected for that task (or the task is deleted), it will error on the next task that has the issue from the bottom order of the WBS.
If you are on 14.4 or 15.1 of the MSP New Driver, see the following KB for a defect you can run into:
KB000007143