Probably caused by the wither sickness layer. There's a "Tag witherstormmod:wither_sickness_immune used before it was bound" string in a generated crash report in memory.

More info on nonamecrackers2/crackers-wither-storm-mod#406.

This is definitely on me, rendering mobs when no world is loaded is never a good idea, other mods cannot really be prepared for this. The 1.18 port of the mod therefore moves the entities to the pause screen.

Will there be a setting to display them in the title screen too?

Probably not

Probably not

I have an idea. Maybe you can disable the main menu entity display when the game is firstly start, but if players come back to the main menu from a world, which means entities are definitely already loaded, the main menu display can be enabled.
And you can add a config to control the main menu display should always enable, or only enable when a world is loaded, or always disable.