Tiger Skull + Frenzy

[dense quiver]

Adding to Tiger Skull causes it to fail when applying the subsequent stacks.

This is likely because it fails to identify which allies to buff once the original target is dead.

I believe this is a bug, since similar "use up card to do effect" actions like recycle still work correctly with

[hollow crest]

Uses Tiger Skull, kills and applies effect
Uses Tiger Skull again (due to frenzy), no ally in the slot to sacrifice, does not apply effect

Not a bug, same thing applies to stuff like frenzy forging stove

[dense quiver]

I feel like this should be consistent with how recycle works.
While your explanation makes sense, personally, I believe this should be changed.

Do you happen to know what happens if you use the Gnome charm on Tiger Skull?
Does it kill two random allies in the row?

[hollow crest]

It should, yes

[hollow crest]

Recycle seems to work on the basis that it takes the cost at a flat value and then plays the card afterwards however many times