On Realms (and likely, BDS), resource and behavior packs behave differently than in other singleplayer and multiplayer worlds.

Steps to Reproduce:
1. Download Mob Heads, Mini Blocks, or Armor Stands resource pack from https://foxynotail.com/addons/
2. Add pack to a realm
3. Join realm using an Xbox, downloading the resources
Observed Results:
Joining freezes on the building terrain step, forcing the console user to close the game and restart, then clear the cache and join the realm without downloading resources. 
Expected Results:
User should be able to join the realm like normal.

The issue at the root of this is a difference in behavior between Realms (and possibly BDS) and local worlds' handling of packs. Other users have reported similar differences in behavior with packs such as DrAv's One Player Sleep pack, where it worked locally but failed when uploaded. Something about this difference causes the packs listed above to fail on realms, causing a crash for Xbox users. 

I have briefly discussed this issue with CornerHard. It worked on his internal tester, but continued to fail in practice on my realm.