We are also experiencing this bug on iPad, where the world template only shows UPDATE, specifically with the DRAGONS++ world template by Spark Universe. We have tried all the usual tricks like clearing space, forcing the iPad to restart, etc. We tried removing the world template from the Settings menu and re-downloading with the same results. I opened the Files app on the iPad and manually removed the "snttYo5wAQA=" folder from Minecraft's "internal" directory and that didn't do the trick either.
I also reached out to Spark Universe technical support today in hopes they will have some ideas. My daughter has many other world templates on her iPad that run fine (sometimes after clearing space or turning the draw distance down), and so far this is only this one that refuses to run at all. Could it be that it was not compatible with iPad? There was no indication of that in the Marketplace description if that is the case.
Just for grins I tried to start the DRAGONS++ add-on on the Xbox (also updated to 1.18.31, like the iPad was) and it just says UPDATE, even after going through the update step, 2 of 2 importing, etc, still says UPDATE just like the iPad does.