The Tavern

I understand certain mutations are suppressed when transformed into other forms and some are not.

I am currently playing a demonspawn in Trunk (0.11-a0-2067) and have the negative energy resistance demonspawn mutation.
Like my rough black scales mutation, the negative energy mutation is suppressed in Spider Form, whereas my attuned to shadows mutation is not.

I wouldn't expect an innate resistance to negative energy to be suppressed in alternate forms.
Is this something that was missed or is it intentional and why?

The list of suppressed stuff seems fairly arbitrary, but some thought went into it. The decision to suppress or not suppress was mostly made for classes of mutations IIRC. rN is a resist, and resists are suppressed (because they change your skin or flesh in some way, and those are completely changed by forms).

But it's resist negative energy. That doesn't seem physical in the least.

I think it would be helpful if there were a clear guide of what's suppressed and what isn't.

Having that on hand in-game would be considered too spoilery as it would list all mutations.

I don't mean in-game. I mean written out so we can see the broader picture and have a discussion about what's permanent and what's suppressed (and when).

I guess that's someone with time and the game wiki, then.

The list already exists in the source. The third boolean (last entry in the first line) for each mutation in mutation-data.h says whether it will be suppressed or not.

It's not clear for all mutations whether they should be suppressed or not. Note that if it could be done either way, suppressing them is just as good as not suppressing them. Also, if a bad mutation is suppressed (or not suppressed), the corresponding good mutation should also be suppressed (or not suppressed) if there is one.

The second boolean is used for Vampire hunger-based suppression by the way, but I'm not sure in what way. Also I'm not sure all forms suppress mutations (lich? statue?).

I just seemed silly to me to repress negative on a spider form.
Dragon breath? Sure.
Scales? Maaaaaybe.
Acidic bite? I guess it's not suppressed.
Repulsion field; unsuppressed since it's a generated thing.
Protection against Negative energy... suppressed? I guess it's some sort of skin goo that deflects draining attacks?

I'd assume that even though some of the suppressions don't make good sense, its required to keep things balanced.