I called Apple support regarding the drafts not saving issue a week ago, and they had me install 3 profiles on my iPhone, do a screen recording of the issue, and he ran diagnostics.
Just heard back, with what appears to be a work around fix. It worked for me, even after shutting down and restarting iPhone.
Open photos, tap select in upper right corner, select a photo, tap share (up arrow in lower left corner), use mail, then tap cancel, save draft. Went to mail draft folder and draft was saved.
Went to mail app, saved a draft, and the draft was saved in draft folder. The support person I spoke to said saving a draft from the photo app apparently created the code that the mail app needs to save drafts from the mail app.
He's communicating to engineering, so hopefully this will be addressed in the next update, but for now, those experiencing this annoying issue can the use the above.
Let us know if this works for you as well.