Hi, noticed a bug with module inserter around (but not using it). Maybe it's related, maybe it's not, but yours is the only module related mod installed.
Just for your info: https://forums.factorio.com/posting.php?mode=reply&f=7&t=92930

I'm going to say it's not because of Module Inserter. The only way it could end up with even allowing 6 modules is if you had a mod installed that added these module slots, then configured ModuleInserter with them and afterwards removed the mod. I'll take a look what happens then tomorrow, but i doubt that is it, since you didn't use Module Inserter

I bookmarked the topic and will check what the devs say.