Either I overlooked something in my testing, or this is one hell of a ninja-bug.
Or was this known all along?
Early conclusion is that Magic Resistant bugs out lightning resistance:
At the time MR spawns, lightning resistance uses
cold resistance to add the 75 from MR to. So in the case of your Frozenstein, 175 cold resistance was used as a base to add 75 to, for
both cold and lightning resistance. So, after MR was applied, his F/L/C resistances were actually 75/250/250.
After applying level 1 Lower Resist, he would still have 219 lightning resistance. Obviously, this isn't reflected in his description.
This can go both ways however, meaning that this bug can end up in your favor:
If lightning resistance is higher than cold resistance before MR is applied, then it means the bug will make him end up with lower lightning resistance than he otherwise would have.
Example:
Geleb Flamefinger. Ph(50), L(100). Spawns with [5]ES and [9]FE, so an extra F(75).
Let's say he randomly spawns [17]LE and [8]MR, now his F/L/C resistances
should be 75/150/75, but in stead, he actually has 75/75/75. His description will say "Immune to Lightning", but don't let him fool you!
This might still be a premature conclusion though, but so far everything has been checking out.
Also, +1 for onderduiker!
As for the description stuff, Conviction and Amplify damage do not seem to update monster descriptions either. I guess they figured the LCS wasn't enough. Decrepify and Lower Resist do seem to "work", but
something tells me that we should not trust that
Edit: Magic Resistant also seems to weird out cold resistance in some cases. I had a monster with 100 inherent cold resistance spawn with MR -> CE, he stayed immune as expected. But when spawning CE -> MR, he was vulnerable to cold damage, despite "Immune to Cold".