You might want to double-check your display settings, I use a 144hz monitor + a 60hz monitor on the side and I've never encountered this problem before.

sorry for quick blame !! something i forgor to mention is im running tmodloader through proton so i think its just some weird way of how its processes displays messing stuff up; mods good thog !!
https://files.catbox.moe/bcr180.png - both monitors turned on
https://files.catbox.moe/6gzfb4.png - other monitor turned off

ok after a bit more looking i realized that you dont need proton to play tmod (im very smart !!!!( but even after running the game , it seemingly just turns off the mod entirely ? tests in the screenshots attached were done with other mods , but i tried a test run without anything else enabled which lead to same results (also mangohud was lost to the fog :(
https://files.catbox.moe/pgqlaw.png - both turned on
https://files.catbox.moe/arhxqp.png - only 1 monitor on at 100hz

Does this issue still occur without the mod loaded? As long as you have frameskip off and a display that supports >60hz refresh rates, Terraria should "run" at whatever framerate your monitor supports.

Obviously it won't actually be smoother -- the game is locked at 60hz update rate so the extra frames won't display anything new (that's what this mod fixes), but you should be able to see the higher framerate displayed by the FPS counter.

ok so the mod entirely not working was just me being stupid; the default settings set frame skip to subtle and since it reset the config when i turned off proton, i didnt check (whopsise !!!!!!!!!!!!!!!!!!!!!!!) BUT the thing about frameskip making your fps your monitors refresh rate i did not know about (i always saw people talking about it and thought it was just mass gas lighting because it has never worked for me (even when i was on windows (im thought the mod was the one doing the frame uncapping, not terraria itself (( and checking default terraria, it ALSO runs at 60fps when both monitors are on but when one is turned off, of course, it runs at 100

so its just a terraria moment

sorry for bothering !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!