The main grammatical rule is that a title stands in for the thing it's the title of. The magazine is singular regardless of the title being singular/plural, and for that matter regardless of whether the title is even a noun at all! It could be a verb (Shoot magazine) or could be completely ungrammatical.
The case where you might use the plural, is if the title of the magazine was a plural that somehow described the magazine itself or its authors/contributors.
I expect it would still widely be held incorrect to use it in the plural, and that the magazine itself would formally use the singular. But no doubt also some native speakers would use the plural and feel justified in doing so, especially in speech. I'm thinking of something like, "The Proceedings Of The Mathematical Society explore the issue". In effect, you're using the title as a plural noun phrase. In writing therefore maybe technically you shouldn't italicise/quote it, on the grounds that this becomes use not reference. I mean the actual Mathematical Society's actual proceedings.
But even then, if the title is taken to describe the contents of the magazine, not the magazine itself, we use the plural. You'd say, "Amazing Stories was good this month", not "were good this month".
You can also reasonably argue that whether I mean the AMS or a different Mathematical Society, The Proceedings Of The Mathematical Society isn't/aren't a magazine. It's/they are a different sort of publication: a journal. I invite suggestions for better examples to replace it.