it doesn't actually work, though. you can click the reblog icon and be brought to the post window, but when you try to post, the post button will not do anything. I'm assuming that the post button not working is in line with the intent of reblog-locking (so that you can reblog-lock a post if it goes undesirably viral and not have people get around it by reblogging the reblogs with replies) but this can still cause some confusion for people who don't realize that the original post has been reblog-locked.
I'd recommend that if the original post is reblog-locked, all existing reblogs (with and without replies) of that post should have their reblog buttons hidden as well.
You can see a demonstration of this here: https://cohost.org/testchosting-9684/post/1002770-this-is-a-rebug-of-t
Just keying in here, had the same issue, and did some analysis to spot what I could about this behavior. I uh, did the noob move of dropping a support ticket when I should've posted here, that wasn't too obvious. Here's the contents from said ticket:
Yeah, I just ran into this (or at least, I think it's this) on this post: https://cohost.org/spiders/post/3033705-whats-the-point-of-m
That post is a share by the author of a post with shares locked, but the second post shows the share option. A post can be created using it, but nothing happens when clicking on post or save to draft