Thought I might add some tidbits to this post, but I saw that on the Gambler mod page, the author replied to a comment and mentioned this bug when opening a mystery crate of sorts, and the inventory being set to a larger size in Svm or some such.
Then I saw on The Crafter mod page, the author issued a fix for that mod when user has Unheard edition set. Seems to have its own stash defined in the code from what I can tell.
Have not tested if this is the issue (I will tomorrow), but seems legit.
I haven't opened any mystery crates or anything of that nature. I don't even run SVM. I only tried it as a work around for the stash issue but didn't help. Let me know if you find a solution.