no RPM drop on mag check

Our Mooney just came out of annual and now the tachometer needle doesn't move at
all during the magneto check as you switch through R, L, BOTH. The engine does
stop running with the switch in OFF.

I asked my my partner who manages the maintenance relationship to look into it
and he contacted the IA who did the annual. The answer I got was:

"I discussed mag drop problem with the IA: he complied with the mag AD during
the annual stating that he also timed the mag and set the points and would
expect very minimal RPM drop. He also tested the P lead and found it normal. The
only confirmation is turning the key off and the engine should stop, I guess
that was done. Looks like a non issue."

This is a Mooney M20J with the Lycoming IO-360-A3B6D with the "dual magneto",
two magnetos in a single housing.

I will check the POH to see what it says about RPM limits during the magneto
test. I'm not where I can access the POH right now.

My question is: is it OK to not have any RPM drop at all when switching from
BOTH to R or L? I'm vaguely uncomfortable with it, but I can't come up with any
reason it's a-bad-thing.

Thanks.