Okay, so I'm feeling pretty stupid right now, especially since I'm running MC on a seven-year-old dinosaur of a mac mini. Just going to get this out of the way!
I was having this issue with the latest 1.12.1 Forge release too, as well as with mods (downloaded from Curse) made for that same version. My first thought was that maybe my problem was with using a beta version of Forge as well as mods labeled as betas rather than releases, so I went back and got the Recommended (at the time of writing this) 1.12 Forge installer and re-downloaded the 1.12 versions of the mods I was using with a preference for releases rather than betas. My skill level in dealing with mods is probably only slightly above "DURRRRRRRR", so I'm just going to apologize in advance if this turns out to be something completely stupid!