Major Note: I do not own the sonic resource pack or somehow have it downloaded
Major Update: I have discovered that the Geometry fix resource pack (here https://github.com/Arexon/geometry-fix) was the cause which removes the recently added block geometry limitations from the recent Minecraft bedrock update by using the same manifest UUID of the sonic resource pack as apparently, the sonic uuid has special properties that remove the geometry limits because the sonic pack has bigger blocks than the limitations.

While using this addon on a single player world it does not add the sonic resource pack, while it does for realm worlds if you add another resource pack or texture pack after that pack. I guess the real bug is that you probably can use this method to have any marketplace resource pack on your realm for free, so patch that and probably allow modders to disable the Block limitations if they need it for their furniture mods so they wouldn't have to try to circumvent it.

Old report:
Original Discovery:
When I added an addon to my realm (Not a Cause "True Undeads" by TrueCowboy) that my friends and I play on it for some reason made me download a 16.7 MB texture pack, and now it appears the sonic texture pack is added to my realm permanently, I have tried to go to the settings of the realm to remove it the texture pack and it is not there.

The only way to avoid this issue temporarily is to go into storage and delete the Miscellaneous file which is the sonic texture pack and not download the resource packs when you join. Others have stated the same issue on Reddit on the realms and MCPE subreddit.

r/MCPE Reddit post: https://www.reddit.com/r/MCPE/comments/t8f1fe/my_realms_texture_pack_got_replaced_by_the_sonic/
r/realms Reddit post: https://www.reddit.com/r/realms/comments/syhobb/sonic_texture_pack_bug/

Update: I fixed this by disabling everything (addons and resource packs) on the realm, which allowed me to delete the realm as it locked me from doing anything for some unknown reason. After I reimport the world (Had a local backup, Realm backups don't seem to work to remove the sonic resource pack), I tested this again by using a resource pack to see if this happens to resource packs as well. I tested this by adding "Brewing Guide UI" by By CrisXolt to the realm, and it added the sonic texture pack again