I have two apps and not long ago I used Sheets to open those same xlsx files from dropbox.
Somehow MS Excel (defenitely not my choice) became the default application and when I click "Open with"
the option list does not show up - MS Excel just opens it up automatically.
Sheets is still available on my phone and I can open up xlsx files.
Android 8.0.0
By the way a few years ago I've reported similar issue for txt files and it was addressed at that time.