Those group rides with max efforts are helping you get better at them.
Your capacity to perform (draw down MPA, recover, and repeat) can improve even when TL hours have leveled out.
What is signature decay currently set to under Account Info, Profile tab?
Are you achieiving BTs during those rides?
If not, when was your last BT event? (shown under signature on XO home page)
What type of BT was it? (outdoors, indoors, targeted effort under fatigue or something else)
You also need to be honest about whether it was truly ‘all out’. If you are getting BT or fakethrough without riding to what feels like the point of failure, your signature is likely understated, potentially significantly
Re: your chart, it does look like your training load has reduced a little (still high though), so that could lead to a reduced threshold actually… are you ‘just’ getting better at sprints or also endurance efforts? how has your ‘focus’ changed over that time? (Same progression chart, just choose ‘fitness signatures’ from the drop down, then make sure ‘focus’ is one of the lines… worth showing low, high and peak TL as well as threshold on the chart
Advice still holds, next time you are fresh, go out and do a real hard BT effort (and let us know how you get on)