You are likely using X11. X11 treats all enabled displays as one "screen" and therefore different refresh rates will have issues (as will VRR for example).
Wayland is the way to go, but the NVIDIA drivers are still buggy with Wayland. Pop!_OS currently uses a desktop environment based on an outdated version of GNOME, so it probably won't be amazing under Wayland.
I'd recommend using a distro with a recent version of KDE Plasma as it has non-experimental support for VRR and great support for Wayland. You'll also want an up-to-date kernel and the latest NVIDIA drivers. I recommend Fedora KDE Spin or openSUSE Tumbleweed. Installing NVIDIA drivers is a little bit more involved (search for "RPM Fusion NVIDIA" for Fedora), but very doable.
I personally switched to an AMD GPU because of the issues with NVIDIA, but NVIDIA support is improving so you'll probably be fine.