On multiple SharePoint online sites, when adding people (internal to our tenant) to the comment on a news post, we will intermittently get the error "(User name) does not have permission to this page, so will not be notified about your comment." We can re-enter the same exact user and then it works. This is happening on more than one SharePoint site, to multiple employees who try to comment and @mention and the error intermittently pops up for different user names selected to @mention.
Here's a screen shot of the comment box where I'm tagging users and user Misty is reported as not having permission. (All employees have permission).