I have the same problem discussed in Ticket 3147. (I am
submitting a new ticket because my attempt to add this comment to
ticket 3147 was flagged as spam.)
When the the text of a highlighted row fails to invert from
black to white, it is completely obscured by the blue highlighting.
Unlike the OP, I have not customized any accent color.
Setting MmMessageListColorInversionEnabled to TRUE did not
help.
A clue regarding the problem is that, after I click a row in the
message list to highlight it, the blue highlight appears
immediately, but there often is a one second delay before the text
color changes from black to white.
Equally true in r5968, r5969, r5970 and r5971.
I suspect that the problem arose after a MacOS update rather
than a Mailmate update. MacOS version is Monterey 12.6.8 on an iMac
with Retina display.