I've noticed an oddity with regards to how Sunsteel Strike and Moongeist Beam interact with Beast Boost and similar abilities. At first, I thought this was a result of me fiddling with the external JSON files to give Lunala Beast Boost, but then I noticed a few things while trying stuff out. Here's a list of all the tests I did:
I modified the external JSON files to give my Lunala Beast Boost instead of Shadow Shield, then KO'd a wild Pokémon with Moongeist Beam. Beast Boost did not activate on kill. On that same Lunala, I killed a different Pokémon with Night Daze, and Beast Boost triggered on KO.
I used a Pokémon Editor to give myself a Nihilego with Moongeist Beam and Astral Barrage in order to test this further. Astral Barrage allowed Beast Boost to trigger, but Moongeist Beam did not. I later changed Moongeist Beam to Photon Geyser, and it didn't let Beast Boost trigger either.
I gave myself a Spectrier with the same two moves as the Nihilego initially had. Grim Neigh did not activate when using Moongeist Beam. Doing the same thing with Glastrier (but with Sunsteel Strike and Glacial Lance) had the same outcome, where Sunsteel Strike fails to trigger Chilling Neigh.
While testing things, I was curious about how an enemy's Mold Breaker would interact with Beast Boost. I spawned a Rampardos with /pokespawn and killed it with the above Lunala and Night Daze. Beast Boost triggered, indicating that Mold Breaker doesn't stop Beast Boost and similar Abilities.
This also affected version 8.3.6, but I didn't have a forum account at the time to report this.
It seems like what this boils down to is Sunsteel Strike, Moongeist Beam, and Photon Geyser neutralizing the user's Ability, but further testing with other Abilities is needed to verify this. If this can be repeated, then yay, I found something. If not, It's probably just a problem on my end.