>>>A firmware bug maybe?>>>

Probably not. A lot of factors involved including what kind of device is driving it. HDMI is more of a general purpose A/V standard. DP is more of PC standard. Acer usually only warrants correct HDMI operation at 60Hz & lower refresh rates unless otherwise stated. Very often a specific PC model must be matched with a specific monitor model at certain refresh rates & resolutions. What brand/model number PC is driving your V227Q?

>>>Anyone have any ideas as to why this happens?>>>>>>PC is a DIY>>>

Sorry. Too many variables, esp in a DIYer, to pin it down. Other than to say, as implied earlier, that DP setups tend to play nicer with PCs than HDMI setups. Not always but mostly. There may be exceptions when pairing your DIYer with another DP/HDMI combo model but likely only found by trial & error. That's pretty much how Acer seems to do it when citing exceptions to its warranted 60Hz or lower HDMI specs. Monitors with virtually identical specs but from different mfrs behave differently when operating outside of Acer's warranted 60Hz or lower spec. No Acer explanations. Just this is how it is.