I joined my friend's realm around two days ago. On the second day, it stopped working entirely. I could still utilize it in personal worlds, however. After discussing this issue with my friend, we determined that they did not enable the setting that requires players to accept the Realm-specific texture packs. I also noticed that upon opening the game now, my texture pack(s) have become disabled & I have to reenable them manually. This isn't an issue, of course, but I feel like the two bugs are connected. 

The video I took was too big to attach, so I uploaded it to YouTube here. Please let me know if you would like additional videos, photos, &/or general information. Thank you.