[Haunted Evil Pajamas]

I don't know if this was intended or not, but when you capture and convert Gold Beetle's into mercenaries they no longer have the Rail Shot spell. I have 4 level 75 Gold Beetles who shot me with Rail Shot as I was trying to capture them. Considering we still have the absolute units that are Grindylows available as mercs, I don't see how much worse some laser shooting beetles would be.

How to replicate: Just capture a beetle and see it does not come with a portable laser cannon.

I don't recall if other monsters who become mercs lose some abilities aside from a passive or something in the Grindylows case.
(Personally I love the fact the beetles have this spell, it caught me off guard and I got a huge laugh out of it.)

[WaifuApple]

This is also the case with the wasps and their new ability, so I assume it's at least somewhat intentional. Would be all for them keeping it, though.

[Trexmaster]

This is the case with skills and spells that monsters gain from levels, they lose them all when they become mercenaries.

As to whether it's intended, I'm unsure, but this happens for every mob with things like Fire and Earth on Firespitters (so they can't ignore fire immunity as a mercenary for example).

[Neus]

This is intended. Skills they gain from scaling do not apply when used as mercenaries.