My experience, may be helpful.
I had a fully negative PSMA-PET (F18 Plarify) scan even after biopsy and MRI showed a PIRADS-2 lesion. Biopsy scored me at Favorable Intermediate Risk (3 + 4=7).
It blew my mind that the PET scan couldn't even visualize the presence of known, biopsied cancer tissue.
My RO and Urologist's responses set my mind more at ease:
1) RO said- There are Micro and Macro cancers. The Micro is at a cellular level and too small to be visualized. When there are enough of the cells congregated together then they can be visualized. So, for example, there might be enough micro cells to show up in a PSA test but not on enough to be visualized on PSMA-PET scan.
2) UROLOGIST said- He agreed with the RO plus he added that because the bladder is so close to the prostate and my bladder was not fully emptied during imaging, the urine (which uptakes the radioactive tracers) lights up and effectively blocks the cancer cells from being visible. At the time, I had BPH and, even after peeing just before the scan, there was a lot of urine left in my bladder. He showed me the images and convinced me that it wasn't just a botched PSMA-PET scan.

So I guess I just had unrealistic expectations of the PSMA-PET scan capability.

Best wishes on you and your husband's journey.