That's interesting...having a higher sample size helps. The issue is, there's no way to know if you were actually getting 75hz without doing some testing; to me, it's obvious on my panel now without needing to do any tests, but the stutter test I linked earlier is an easy way to see it.
Perhaps the issue is only with Nvidia cards, and not cards without Freesync. I'm still thinking it's cards without Freesync, but the only way to know is to test.
Another note: using the monitor's OSD, you can check the monitor refresh rate. For me, Freesync was broken even though all settings said it was enabled. No matter the FPS, GPU, or monitor settings, the monitor refresh rate would never change. Apparently, the driver that Windows installs by default (for me, that's 17.1.1) breaks Freesync. A ton of people in Reddit had this issue as well. I updated manually and Freesync works great.
To sum up what we know so far: a) we need more tests with older AMD cards and Nvidia cards,, b) all available information so far tells us that Nvidia cards do not support 75 Hertz on this monitor, c) Freesync seems to be broken with default Windows drivers, if it uses the Crimson (Radeon software) version 17.1.1.