When playing with this mod installed, the tech for Molten Plastic becomes hidden, making the player unable to research it or any subsequent techs.

I don't hide technologies. Is it this mod?
https://mods.factorio.com/mod/molten_plastic

If you want to verify without programming knowledge, you can do a text search on the extracted mod for the word "hidden", it won't show up. That is the name of the field I'd have to set.
https://lua-api.factorio.com/latest/prototypes/TechnologyPrototype.html#hidden

I did go through the mod's files, along with Outer Rim and found it strange that it in fact did not have the 'hidden' key anywhere. I don't know what other mod could cause that, since in game the only mods in the mod owners tooltip of the tech is 'Molten Plastic > Science Spread'.