My threshold power this morning was 273 ish - I commuted to work and forgot to record - so I did a manual entry and estimated stress (the ride was 9 miles) then I rode home in the afternoon - the last part of my ride is all uphill - took a rest day yesterday - so i did some fairly hard efforts on the uphill and got a ‘near breakthrough’ - after I uploaded the ride and my Threshold Power dropped to 266. I am a new user of Xert but not new to training with power
Normal behavior for a near breakthrough. It’s Xert’s way of “dialing in” your signature. Next time do an all-out on the uphill, and see what happens.
A Near BT means you were very close to proving your signature numbers but fell short of a real BT.
If the effort was cut short for any reason (hill ended, traffic, sprint was over, didn’t give it your all) and you are concerned about the signature change, you can flag that activity.
to 
That will exclude it from signature calculation while strain analysis remains to measure training load and intensity.
Otherwise, the idea is to bump one or more numbers down so you can more easily prove a change in your signature values moving forward.
With Xert your fitness signature is in constant flux and recalculated after every activity or lack of activity (decay).
Near BT adjustments are typically minor and result from an intended BT maximal effort under fatigue to failure. A minor change indicates your signature is dialed in. 
In your case I suspect you cut the effort short of failure, but Xert doesn’t know that so the drop was higher.
Reference –
Lots of newbie tips posted here – Beginner questions
Academy series videos are the quickest way to get up to speed – Discover + Improve + Perform
FAQs here – Support Home – Xert
Welcome aboard. 