1) I'm using the mobile version.
2) I'm uploading the image directly.
3) Galaxy S9+ Android, Chrome V 81.0.4044.96 (I just updated and retested before posting this)
So - one possibility (without me breaking away and spending a ton of time investigating this), is that the image you're attempting to upload is too big. There is a cap of 500k for image sizes (and even that may get reduced in the very near future). If you are using the desktop version, uploading a file greater than 500k will cause a "Your file is too big!!" message to appear and won't allow you to upload - unfortunately, I was not able to get that same message to appear in the "mobile" version of the editor (this is a 3rd party app, using 3rd party plug-ins - so there's only so many weeds I'm willing to stumble through, for something like this!!!). Even without the message though, the cap does remain in place - so attempting to upload a file larger than 500k might appear to be successful, but the storage function will simply drop that image if it's too big...
...can you confirm the size of the image you're attempting to upload? If you have access to a desktop (or can hit the "display as desktop" link at the very, very bottom of any page when viewing the site on your mobile device so that it can emulate "desktop" on your mobile device) - can you try to upload the same image using the "desktop" editor?
Let me know how you make out...