I have this exact same issue with the factory grips and factory mags on my TS .40.
I've broken mags down a few times for cleaning and it still happens every now and then.
It doesn't always happen, but when it does the slide doesn't lock back even with an empty mag inserted.
One thing I found is that when the issue arises, it is usually during dry fire practice. When it occurs I notice that the slide lock/release is slightly out of place. If I remove the mag and push the slide lock all the way in, the problem is resolved and mags will drop free again.