I am not sure if this is specific to the world I am in as I am playing in a modified superflat world, but there are multiple chunks that minihud shows as not being slime chunks when they are. I have seen slimes spawn in those chunks and I have confirmed with chunkbase that they are, in fact, slime chunks. I am on 1.20.1 and using minihud version 0.27.0.

I assume you are in single player? It's possible that the mod has stored an old/incorrect seed for the world. If you have commands enabled, then do /seed and MiniHUD should grab the correct seed. If not, then you can also input the seed with a chat message like minihud-seed 12345. See if the overlay changes after you update the seed.

I probably need to improve the seed handling somehow... Because currently it's possible that the mod may use the wrong seed if it stores one seed, and then the seed "changes" somehow. For example if you log into a world, then delete that world, and create a new world with the exact same name, the mod would then use the same per-world config still and that would have the old seed stored. I should probably always read it from the world when you log in to a single player world.

That seems to have somewhat fixed it, as some of the chunks that I know are slimechunks are now registering, but I have no clue how that had happened in the first place. Weird.
Also there still seems to be a few chunks it's not considering to be slime chunks that are, although I can't confirm that as of right now.