I have one additional piece of information that may be relevant.
After checking my email more carefully, I found that on June 26 WordPress sent me a Recovery Mode email reporting a fatal error related to Jetpack.
However, I never noticed any visible issue on the site at that time. Both the frontend and the WordPress admin appeared to be working normally, so I didn’t realize an error had occurred until I found the email later.
The error reported was:
E_ERROR in:
wp-content/plugins/jetpack/vendor/jetpack-autoloader/class-php-autoloader.php
Failed opening required:
jetpack_vendor/automattic/jetpack-connection/legacy/class-jetpack-tracks-client.php
The stack trace points to Jetpack’s tracking/connection modules while loading Jetpack.
I also performed one more test today:
Switched from my BuddyBoss Child theme to the default Twenty Twenty-Five theme.
The WordPress.com Notifications icon was still missing from the admin bar.
Switched back to BuddyBoss Child afterward.
So it does not appear to be caused by my theme.
I’m not sure whether this Recovery Mode error is related to the missing WordPress.com Notifications icon, but the timing is very close to when the problem first appeared, so I thought it might be useful to mention.
Thank you!