I can confirm that currently the life orb is dealing too much damage to a dynamaxed pokemon holding a life orb. Bulbapedias page on the life orb
states that "For Dynamax Pokémon, the damage is equal to 10% of the Pokémon's non-Dynamax maximum HP." - currently in pixelmon 9.0.10 the recoil damage is 10% of the new, dynamax HP.
I've moved this information over to the internal tracker to be resolved - thanks for the report!
P.S: I've removed the security status of this bug, so others experiencing the same issue know it has already been reported. The security ticket feature is normally reserved for bugs that are highly exploitable or can crash servers.