provocative but not explicit and some overly “friendly” messages. Things like “makes my day when I see you” “you look good today” “when are we going to have dinner”
He's also gas lighting you saying you've misunderstood/misinterpreted the situation.
If he saw this between you and another man, how exactly would he interpret the situation? And don't listen to any lies he tells of that question if out to him.
They are, at the v least, having an emotional affair.
And you say it's not his first.
Whether he has sex or sexual contact with them or not (and it's unusual for there to be no sexual interaction or contact), you should not have to be subjected to emotional affairs by your life partner. He's supposed to be your partner. Would he put up with it from you?
(Even if he would, very unlikely - that still wouldn't mean you should put up with it).
It's in opposition to everything that a partnership is supposed to be.
"Makes my day when I see you" is not something an attached person should be saying to anyone but their partner.
And it's not platonic (which would be tremendously ott if platonic anyway) because of the provocative pics and physical compliments etc).
It's equally inappropriate to be arranging to meet one on one for date type activities - when the above is the case.
You aren't misinterpreting anything.