I have just recently encountered this bug. This started at 11am (UK time) 9th October 2020. I was going to do a checkup on my realm World 2020 and it was loading for quite a while (stuck in the "Building terrain" stage- The final stage. I then gave up and played CubeCraft. I can also join other realms and servers and have no problembs with the "Building terrain".

I had lunch and then tried agian. I waited for 5 minutes and still nothing. I refreshed Minecraft and tired agian this time I recorded it. And it was so long at the "Building terrain" stage that I was able to do a 10K bike ride (not exaggerating). It took 50 mins and I then gave up.

I did a world download to check if any hackers or griefers made a lag machine or made a way to prevent admins from joining and there was nothing. Is it because of how much storage the realm is useing up (My realm consumes 350+MB) or is it because of how many players are on. This will also lead to another bug where owners of a realm; I will not show how many players are on (always show 0) it dose sometimes show how many players are on. When I was recording, only 5 players were on. 
This bug occurred on my IPhone and it might also happen on my Xbox. I am very consurned and worried about my realm as I am the only admin and only a few Elite and normal 2020 troops are active (police force for my realm, they patrol my realm when I'm offline. I did say to one to close the realm in game if I cannot join in general. Please fix this bug