I have been having the same issue with the realms addons I spent 5 hours trying to upload a pack to my realm and going through the uploading process the way it is supposed to work was causing me to be stuck in the same repeated loop as mentioned previously, I tried both methods by downloading the world and uploading it to the realm through replacing and I found a way around this bug. I was able to eventually upload my addon to the realm by making a copy of the realm and completely resetting the world in the realm's world selection converting it back to a fresh world. I then added my addon to the copy I made of the world before resetting the realm and replaced the newly reset world with the one with the addon. this way of doing it though a longer prosses has worked for me a couple times now.
I have been having the same issue with the realms addons I spent 5 hours trying to upload a pack to my realm and going through the uploading process the way it is supposed to work was causing me to be stuck in the same repeated loop as mentioned previously, I tried both methods by downloading the world and uploading it to the realm through replacing and I found a way around this bug. I was able to eventually upload my addon to the realm by making a copy of the realm and completely resetting the world in the realm's world selection converting it back to a fresh world. I then added my addon to the copy I made of the world before resetting the realm and replaced the newly reset world with the one with the addon. this way of doing it though a longer prosses has worked for me a couple times now.