JdeBPI think that the telling diagnostic here is that in an otherwise identical setup, for a single-monitor system and the old HP monitor type, you replaced one monitor that worked with a supposedly identical one, and that second one did not.
That's highly suggestive of those two monitors no longer being identical, where once they were.
Do they send the same EDID/DisplayID information to the system?
Have you configured any EDID overrides for the kernel, with custom EDID files?
When you bring up the monitor's own OSD, what does it say about itself?