Noise from engine I believe that has seemingly gotten worse, looking for some help figuring out what it is

I've got a crf250r I'm trying to access the top end of the engine which i think is the reason the engine is full seized up though the steps in the manual say that i should "Make sure the piston is at TDC on the compression stroke" when removing the cam shaft. though i can't do this cause my primary drive gear is locked up. So my question is do i just avoid this step?

I have a DT200WR fitted with a CDI and YPVS module from a DT200R.

The problem: at low-to-mid RPM the YPVS works fine, opening normally as I rev up. But after a certain RPM, right as it reaches maximum opening, it suddenly closes completely.

What I've already tested and ruled out:

• The module and servo themselves — I bench-tested by feeding the module a simulated RPM signal via Arduino, and it opened perfectly up to 9000 RPM with zero issues;
• Swapped the CDI for another unit — the exact same fault persists;
• Servo gear ratio — swapped it out, behavior unchanged.

One important observation: measuring the RPM signal wire going to the module, I noticed the AC voltage drops by about half at high RPM — right around the point where the YPVS starts closing and oscillating. It starts at 300VAC and ends up at 180VAC.

Since the fault only happens with the real signal from the bike (not the simulated one), I'm out of ideas as to what could be causing it. Has anyone seen something like this before?