Are you talking about an image field or a gallery field?
When you are testing are you logged in or logged out?
I just did a quick test with an image field from an android phone in FireFox browser and was able to successfully upload an image. The only issue I had was that the “upload” tab was sorta missing when the WP image select screen appeared, but I was able to tap to the left of the media gallery tab and the upload screen appeared.
