I've just upgraded my Pixel 3 to Android 11 and rooted it with Magisk Canary built from revision e66b0bf3 (github.com/topjohnwu/Magisk/commits/e66b0bf3b2ac1fc95ccc6cebe3d01e455908882a) which includes the commit you mentioned above and has the specific line you noted (github.com/topjohnwu/Magisk/blob/e66b0bf3b2ac1fc95ccc6cebe3d01e455908882a/native/jni/magiskpolicy/rules.cpp#L106), but I'm still getting the message and no recording is made.
The full message is: "You enabled root options in Boldbeast Recorder, but the app cannot get the su privilege. Please grant root access to the app, otherwise it cannot apply recording parameters."
Like OP, I removed it from Magisk Manager, let Boldbeast Recorder ask for it again and granted it.
During calls I see a notification that recording is in progress, but when the call ends I get the message from above.
EDIT:
I tried disabling and enabling root options in Boldbeast Recorder. Now I can't get them back again. When I click "Enable Rooted Options" I see the notification at the bottom of the screen saying "Boldbeast Recorder was granted superuser rights" but after a few seconds Boldbeast Recorder itself says it didn't get them and proposes I should root my phone.
Within adb shell I do get root access when I run su and can access privileged directories. Since su works from adb shell this feels like more of a problem in Boldbeast Recorder perhaps?
(I can't post links so add the dot in "githubcom" where necessary.)