Hi Jay,
I managed to resolve this by going into game settings from the world selection menu, hitting the multiplayer tab, toggling the "multiplayer game" toggle off then on again. I wear I tried this before reporting the bug and it didn't work but now it's resolved.
I might be able to pull a copy of the world out for you. I was using a Windows virtual machine so I'll boot that up and see if I left the files there.
I’m also experiencing this issue on 1.5.3 with switch. I’m able to upload my worlds to realms however but if I download that world on another platform it has multiplayer disabled.
What I did was turn on the resource pack for the server curious what my world looked like. I got the warning but it did nothing to suggest that cross play would be disabled forever. When I turned the resource pack off cross play invites were still disabled.
I also did not use the mario mashup world template. This is a world I created with no texture pack or template.
I have the same issue on macOS 10.14.5