Let's try to figure out if the issue is with message states (the app thinking the messages are read), or with how they are displayed.
Is the background of unread messages different from that for read messages? In the light theme, that would be pure white vs. grey.
Does the folder show an unread message count?
If yes and yes, then it's how the messages are displayed.
And guessing further -- is this a Samsung device? Did you select a custom font?
Some of those custom fonts (I can speak for Rosemary, maybe others too) does not have a bold version. So even if my code asks for bold, it comes out "regular".