PRI interrupt triggering when VSYNC "syncs" normal interrupt

[arnoldemu]

Two HSYNCs after the start of VSYNC the CPC synchronises standard raster interrupts by resetting the counter.

If I cause a PRI interrupt to trigger at this time I get a unexpected result. The PRI does trigger but it seems to be around ~300-400 lines later depending on the value in R4.

At other times during VSYNC PRI will trigger fine and on the expected line.

I need to do more testing to confirm what is happening here.
I set R4 and R7=0, then set PRI to 1.