I have the same issue here. I don’t understand the answer, if it gets flagged, it gets flagged and may not make it to the inbox. Even if it isn’t for a good reason.
I just installed Mailpoet 3 – never used it before. it is not showing up on my dashboard. I’m running WordPress version 5.0.1 and PHP 7.0.31.
I found a workaround. I was using the ‘Blank’ template in The Events Calendar settings, part of the HighendWP theme. When I switched to the ‘Default Events Template’ the issue went away. I just had to adjust a few css values to get it to look like I want and problem fixed. I’d still like to know why my add_filter code wasn’t working however.