All add-ons and resource packs are up to date. After attempting to load onto a realm with more than one add-on I found missing icons and error tags. I found that deleting cached data randomizes the errors. Also, a realm with anything over 355~Mb (including resource packs) causses a crash. Players who have installed most or all these resource packs and add-ons natively don't have any issues (assuming because they wouldn't be cached the same way). Deleting the incorrectly cached addons in a random order will eventually resolve all errors.  When incorrectly cached the realm will not reattempt to download missing files even if only the indicator saying it exists is the only thing there. Findings, despite seeming random, are fairly consistent in replicability. In the final case, Spark Portals outright deletes and replaces Decocraft but upon reentering the realm it makes no attempt to recache Decocraft now that it is gone. If I had to a take a guess, this is the symptom of a larger issue with add-ons and how Minecraft installs dependencies when joining a world with them. And just to restate, the host has no errors on their end.

Contents tested:

Kingdoms v1.0.15 (installed), Kingdoms v1.0.15* (cached) 

The following are all cached and show no data beyond their size and contents:

Decocraft Base RP*, Security Add-On, Farming Addon, More TNT! Add-On, Magic Spells Add-On*, Gravestone Add-On*, Another Furniture Add-On, XP Crystal Bank, World Utilities Add-On, Spark Portals*.

• there is some kind of error resulting from not being cached correctly