The way I understand it the PETPSMA scan combined with PSA test results give indicators of what’s going on with metastatic cancer. Unfortunately, you can have rising PSA (small values) and not see any new sites on a scan.
Thankfully, technicians are getting better at reading these scans and interpreting them and should be able to weigh in with some helpful information when comparing a current scan and a prior scan.

Good luck to you on your journey.