-Hi, my name is Claudeir, I'm happy to help you!
This is totally normal behavior on some monitors that only support 75 or 60Hz, I myself have a monitor that with the lower resolution, I can use 75Hz, but with the native resolution, I can only use 60Hz, this is normal, it would be abnormal if the If your monitor were 144 or 240Hz, if they had these Hz ratios natively then they should support it regardless of resolution, whether it's 4:3 or not.
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Graciously,
Claudeir