If you sign back into your email and open a new Compose page, does the email appear?
When in the webmail interface on a desktop or laptop computer, there is an auto-save function running in the event something happens while composing a message like a power outage, or maybe you closed the tab you were in by accident etc. However, as far as I know, it does not work on a small mobile device like a phone. It doesn't appear on my iPhone. But it does work on my laptop using a browser and the webmail interface. Other than that, I have not encountered what you described myself.
I was able to reproduce the same problem on my Android phone using Dolphin browser. My browser's user agent was set to Android, which displays web pages in a mobile version.
I changed the user agent to desktop, which changes the display to the full version. When I refreshed my email, there was a link in the lower left of the inbox showing the subject of the incomplete email. I tapped it, and the email opened. But since the desktop version doesn't display in a user-friendly format, I was't able to work with it. However, the discard button was visible enough to tap. When I tapped it, I was given the option to save as draft, which I did.
I then changed the user agent back to Android and refreshed the email page to go back to the mobile version. I was able to open the incomplete email from the drafts folder. From that point the email could be edited, sent, discarded, or saved as draft. Note that the option to save as draft appears after tapping discard.
I wonder if there's an easier way around this. 🤔