How many Breakthroughs (BTs) appear in your three month history?
There is a relationship between signature values and if you have a wide jump in Peak Power (PP) that will cause a recalc of TP.
A sudden jump in PP can be a power meter issue especially during an outdoor ride.
You can always flag a BT activity when you don’t agree with the assessment.
That retains the strain score while nixing the signature calc.
Did you ride the Fitness Test workout entirely in Slope mode?
See: From old trainer to smart trainer. Set FTP - Training - Xert Community Forum