Q: weird behaviour of flags in Mail.app

Hello there,

Honestly, I don't even know how to describe this bug. It's simply weird. If you look at the picture bellow, it will hopefully make more sense.

The context:

• I am using POP3 accounts exclusively, there is no exchange server, no synchronisation with an iPhone or anything fancy.
• My OS version is El Capitan 10.11.6
• My machine is a Mac Pro mid 2010, I've started with Snow Leopard so I've done quite a lot of upgrades
• The problem seems to have appeared after the upgrade to El Capitan. I have some other issues with the Finder (that co-workers don't seem to experiment so I don't know if there simply is something wrong with my installation or if it's a Maill.app issue.

The bug itself:

• I am in the Flagged > Red section, yet I can see emails with gray flags in this section
• Emails with gray flags in the Flagged > Red section are not the same emails as the one in the Flagged > Gay section
• The reason why I've started using gray flags was because I couldn't remove red flags (or the email would stay in the Flagged > Red section, unflagged). At first, emails with gray flags would migrate to the Flagged > Gray section but they don't do it any more and, of course when I unflag them, they also stay where they are.
• Fun fact : if I unflag the emails in the Flagged > Gray section, they disappear, the counter disappears, but the Flagged > Gray section still exists (it's just empty)
• The email count of the Flagged > Red section is not constant and doesn't always make sense. Sometimes it displays 33 (which is the number or emails with red flags), sometimes it displays 57 (which corresponds to nothing because there are 63 emails with a red or gray flag in this section).

Thanks in advance for your inputs