Hi there,
I don’t .
I do NOT think it’s an Rserver BIOS BUG issue, I know this must pain you to hear that(flag), But had you done a quick Google Search, You will see it’s also the case when connected to other systems. So very much on “TOPIC” It’s could be you monitor’s refresh is off or failing to switch DOWN to lower refresh… You even indicate it works with other monitors.
I see other users complained about the “75Hz refresh seemed to have little effect on work use and only works over DP, not TB.”
HTH
Enjoy GL 
PJ 