We are aware of an issue that has been introduced with MSFS Sim Update 15 that is causing MSFS to trigger an "Overstressed" warning when the storm window is opened in flight.
We are currently investigating this and we aim to include a fix in a future update to the PA-38 Tomahawk (MSFS).
In the meantime, the overstressed warnings can be disabled at MSFS Options > Assistance Options > Failure and Damage > Aircraft Stress Damage.