Yeah! Because it is a WP plugin!
You have to install the phpBB extension, if you like to have users registered/added into WP at same time when they register into phpBB.
Or an user that register in phpBB, will be added in WP, only at first login in WP, or when as logged in, will visits the WP side.
phpBB common tasks extension (1.0.2 add users at same time in WP when they register in phpBB, and with redirects and options you can activate):
viewtopic.php?p=5509#p5509
p.s
note that if using iframe, the option do not need to be used (nor redirects, read hints). In this example, you registered into iframe: as soon you login, then you are added into wordpress, because the page reload, a valid phpBB cookie exist, and the addition fire (when you have been redirected to wp home page at first login). Note that now, if you logout and re-login, you are not redirected into WP page, but correctly here into page forum if you login into iframed phpBB. This could/would happen also at first time login, but to resolve a bug that cause a duplicate user insertion (anyway after fixed) the redirect is intentionally to a non page forum, when the first login happen, and the user being added also into WP.... so hard, and not too complete explain, i know, let resume that it is this way because yes
read all info hint on plugin admin (i know, my Eng discourage to do so
)
and
https://www.axew3.com/w3/wordpress-phpb ... n-install/
you'll see that you can do anything you want, and more!
Follow with any question if in trouble
Going to test an install as you describe, with any user's flavor, may something still need to be fixed, and will be in case on coming very soon 2.6.0