My type 4 Radom has a slide lock issue too.
If hand-racking the slide, it locks back with a mag in.
If the mag is ejected, it will unlock the slide.
If no mag is in and the slide manually locked back, it will self-unlock after a half second or so.
When shooting, it won't lock back on empty 70% of the time as apparently the recoil is severe enough that it overcomes the follower spring.
I came to the conclusion that the slide catch slot wasn't cut square enough (like kman).