My story is a little similar although my PSA started up shortly after the RARP. I believe the theory is something like,

"there's obviously some cancer somewhere shown by the PSA rise, since we can't find it elsewhere, it's likely a small amount of cells in the pelvic area still."

My PSA was pretty low and PSMA PET was negative as well. I have an older chart posted by one of the guys here more educated on the subject, it indicates that with a PSA of less than .5 the detection rate is only around 38%. (this chart was produced in 2019). It's floating around here in a couple places but I'll post it again.

I did 39 treatments 30(ish) to the "pelvic area" and the last nine(ish) to the prostate bed alone. I also did two years of ADT (orgovyx). So far so good after three years post radiation and 15 mos post ADT.

Best of Luck to you!