This sounds very similar to what I’m experiencing. I began off season after my final XC race in late Oct.
I took 10 days almost totally off the bike. Then began heavy to me weight training and core work 2X per week, and consistently added weight to the lifts. During this phase I did 3-5 endurance rides per week, but overall a fairly large decrease in volume and no intensity. Other than the few indoor rides I wouldn’t even pay attention to power or HR data.
Now I’m starting my annual training plan and definitely lost some bike fitness. FTP dropped from 304 to about 270. While it’s not desirable I’d say this was the intended consequence of my plan. I made a lot of strength gains and lost bike fitness by taking a break from structured cycling and volume.
Now I’m back to structured rides and decreasing the weight sessions to once a week. Also adding one yoga session per week.
I think our fitness will return quickly after a few weeks of structured training, but we’ll also have the benefit of the strength training.