A wordpress password hash cannot be recognized by phpBB, a phpBB hash cannot be recognized by WP, if you do not provide an easy extension or plugin, that can check for it's validity in phpBB when an user login, or viceversa: phpBB and wordpress uses different ways to hash passwords.
And let me say that i do not understand why WordPress do not use the default PHP passwords and still use an old not useful library.
https://www.php.net/manual/en/function. ... d-hash.php
It really result absurd to me. If anybody have an idea of the why, please let me know.
Anyway, returning to your question:
A phpBB user transferred into WordPress, until the plugin is active, will login into WP without having to change any pass, because the integration plugin check the password in several ways, and also like phpBB do.
If you transfer WP users into phpBB, instead, they are transferred, obviously, with the WP hash, that phpBB do not recognize.
So at this time, until users will not have reset their pass, they will be able to login with the same old password only into WP and not in phpBB (until a reset pass do not happen).
Until the plugin is active, it hash the WP/phpBB users password in the phpBB way (
https://www.php.net/manual/en/function. ... d-hash.php) but it recognize at same time all php hashes, also the old md5 and the phpAss default library of WP.
At the time that the integration is disabled or uninstalled, you can expect this:
phpBB users will have an hash that phpBB recognize, so phpBB users will be ok: but those transferred from WP to phpBB, only if them in the while changed their password in some way, updating it along the integration time (because phpBB do not recognize WP hashes).
WordPress users, once the plugin will be disabled, will have to reset the password because the used hash is not recognized anymore by WordPress (but a single plugin done of +-50 lines of code can fix the problem, whenever required/needed and i will provide one asap).
Also, the phpBB extension could easily be improved so to have an option that if active will recognize in phpBB the WP hashes.