When breeding a Pokemon with a regional form, there is no way to choose what form you get, it only gives you the base form. In this specific case I was trying to breed a Galarian Yamask. I used the following combinations:
x2 Runerigus
Runerigus + Cofagrigus
Runerigus + Ditto
Runerigus + Koffing

In all instances I tested with no everstone, an everstone on the male parent, an everstone on the female parent, and an everstone on both, with no difference. The wiki mentions choosing the regional form preferred via "yellow arrows" in the Day Care UI, but I don't see them at any point while setting up the breeding process.

Hi There!

Thanks for reporting this! I was able to go ahead and personally test this out myself and can confirm this is a bug! Moving to internal.

Adding to this, it's not just Runerigus.

Pokemon that evolve exclusively from a Regional variant are not technically variants, and therefore produce the original form of the base Pokemon when breeding.
These Pokemon are:
Perrserker
Sirfetch'd
Mr. Rime
Cursola
Runerigus*
Obstagoon

The Hisui pokemon don't technically breed, but should result in their hisui variant
Overqwil
Sneasler
Basculegion (should go to white imo, but white isn't technically hisui variant)

by "hisuian don't breed", I mean in PLA, not in Pixelmon