Thank you for your post.
I have been trying to duplicate this problem, but not having much luck.
There was a similar problem reported prior to the 10.0v3 update, and that was fixed in v3. This is what I have done.
I have FileMaker Pro hosting a file with two full access accounts. The host is using the default Admin - <no password> full privilege access. The guest is using FileMaker Pro Advanced and opens the file remotely. First, with a second user account (TSGal - TSGal) with full access, where I'm able to change the password (TSGal1), close, and reopen with the new password. Then, the file was opened remotely again with the default access that the host was also using. I change the password to "admin", close the file, and reopen with the new password. I even set these to change password next time I log in, and that password change is valid the next time I log in.
Let me know what I'm doing differently than you, as I'd like to duplicate the issue and report it.
I've tried to replicate the behavior I reported, but today it works as expected and don't see what I am doing differently. Perhaps I clicked a cancel button instead of OK or something equally silly.