Looks like a case of not enough data after the BT event.
There are similar posts from others whose BTE occurs at a point at which they stopped pedaling afterwards which is where the algorithm relies on more data to establish your performance while fatigued.
If you don’t feel confident about the changes you can flag the activity to remove it from signature calculations.
For more accurate BT calculations the activity should include a range of power curve data as shown in BT workouts (search library for “breakthrough”). Or outdoor rides that include a similar range of efforts and fatigue before and after the BTE.
For BTE examples (including some problem ones) see this thread as well as the blog article linked below for best ways to achieve BT’s that dial in your signature.
Let’s See Some Recent BT’s - General - Xert Community Forum (xertonline.com)
Reference –
Breaking Through the Xert Way! – Xert (baronbiosys.com)