what happens if CS/P5 welding was done with CS filler ? 1

The P5 max PWHT temp will most likely lower the A1 chemistry weld filler metal below its minimum specified tensile strength. However if both base metals have a MSTS of 60 ksi the PQR can meet ASME IX requirements. It is noted that the max PWHT temp of P5 will be above the lower critical temp of both the P-1 base metal and deposited weld metal. One should therefore limit the max PWHT to 1340 F, the lower critical temp of CS per B31.1. We have recorded tensile strengths of ER70S-2 and E7018 with 1325 F PWHT between 64 ksi and 68 ksi multiple times. We have also recorded tensile strengths of E7018-A1 above 70ksi at 1325F PWHT.

Thanks weldstan, that kind of information is gold to me.
If/when 70ksi cannot be achieved, there is a workaround but it onerous.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

I generally specify E7018-B2L and similar weld fillers to make this weld.