Just some other checks as you said that disconnecting the IR board stopped the problem and the TV remote tests OK.
Do you only have the one remote for the TV or are there other IR remotes in the room such as a Firestick remote etc?
If others test them as well or remove their batteries and check if it still happens.
If no others, temporarily block the TV's IR window to prove whether the problem is internal to the TV or caused by external conditions.
I've had a case where a faulty CFL ceiling light was triggering a TV's IR sensor every time it was turned on.
If internal here’s a link to the service manual for the TV. (be patient it takes a little while to go from “processing” to Download”).
It shows that the IR board and the Power On/Off button are in the same cable to the mainboard. It would appear that if the replacement IR board also doesn’t fix the problem then there is either a fault in the cable to the mainboard or in the mainboard itself
On p.29 it shows the part number for the main board - TZZ00001035A which appears to be out of stock in most places that I’ve searched when using the part number. You may have better luck.