This issue happens with m3 power block, yes it happens with older varients but with only first few shots. Fx keeps missing around with every new variation of the impact.
I had this problem and solved it by polishing the valve rod and valve puppet with a very high grit. Now even first shot is at normal set velocity. I used dremel to rotate the rod and used cloth polishing materials.
Your main doesn't seal probably so the air in the small passage between the two regs slightly creep to the plenum. If your gun is new let it settle, if not then you need to rebuild the second reg and check the piston head.
Yes it will be more affected by ambient temperature. You will see 3 to 5 bars difference between shooting in the morning vs shooting in afternoon with 72cc plenum.
The new power block has 3 orings inside to seal the valve housing and one more oring inside to seal the barrel. We need to know their sizes cause I noticed they are easily nicked.