Using this alongside FancyMenu, even if the loading screen customisation is off in FancyMenu just displays the vanilla loading screen. I'm not sure if you can do anything about this tbf.
Fabric, mc version 1.20.1
Seamless Loading screen 2.0.3 for 1.20.1

I dont have this issue. however it doesn't work if you use a continue button to join your last played world

An image of the issue would be great.

Crashed while on pause menu and closing minecraft. An interaction with Fancy main menu
on mc version 1.19.2
Crash log
https://pastebin.com/amxME7gy