I agree with @Ponce-De-Leon, you should copy the icon set to your ~/.icons directory (Create it if it does not exist. Remember, the hidden system directories have the period in front of the name).
Flatpak/Seal will not access /usr/ directory.
Okay, I understand. In the Past, I had the same Issue with that. I had put it in this local Path too. And then I had on some Programs or the Desktop with my chosen Cursor Theme and some not - it was a Mixture.
So I changed the Place for my Cursor Theme. I have put it to /usr/share/icons/ (not to /usr/share!) because when you put it there it is use systemwide. But you need sudo Rights. I think the easiest Way to do it is in the Terminal.
And when You placed Your Cursor Theme Folder in the usr Path make a reboot and then go to gnome-tweaks an change Your Cursor Theme. For me that works. I hope for You too.