I am running an iPad Pro (3rd generation) with iOs 15.1, but I had the same problem with previous versions of iOs as well. I have two issues.
First, I have tried a number of times to add Dropbox folders as "Favourites" in the side bar. It works when I add them, but then later they just disappear.
Second, when trying to access my Dropbox files through the iOs Files app, the connection seems to periodically drop out. It works fine most of the time, but often I will try to load it and it just shows me a blank screen. I then have to leave it sitting there for about 10 minutes and eventually it reloads and takes me back to the home Dropbox folder. But during that time if I try to access Dropbox through the Files app it just doesn't load. Below is a screenshot of me trying to access Dropbox:
