When opening an ItemSwapper screen, which are intended to be used to quickly swap things in your inventory whilst moving, Multiplayer Server Pause incorrectly treats ItemSwapper UI screens as pause menus and pauses the game on a server.

https://modrinth.com/plugin/itemswapper (this is a Fabric mod, the url is due to it also having a plugin version)

Examples of UI which is leading to the server incorrectly considering the player as being in a pause menu