Okay, in the interim I have a shipped a fix for Android (seems fine on an iPhone emulation) that uses two tens like so "𒌋𒌋" (looks like <<) instead of one twenty "𒎙" (also looks like << but a bit tighter). This is definitely one of the weirdest patches [0] I've ever done—changing how an ancient language is displayed based on the specific type of incomprehensibly advanced technology it's being displayed on—but I guess that's what Sundays are for.

[0] https://github.com/OisinMoran/OisinMoran.github.io/commit/15...

Because Google has mostly stopped updating the font stack on Android.

See bug report https://issuetracker.google.com/issues/366415133

Well, I can report that 𒎙 isn't rendering on Windows 10 either. Your "sent at" renders fine. Whatever it is, it isn't specific to Android.

I assume it's mostly down to fonts, but I don't know why a font would implement some of the cuneiform block without doing all of it.

U+12399 was not an assigned code point until Unicode 8. The other code points were in Unicode 7.

I guess the Arch in Arch Linux isn't for archaeology then :(

You'll need to install the `noto-fonts` package to get NotoSansCuneiform-Regular.ttf (amongst others)l I'd recommend also installing `noto-fonts-cjk` to enhance your web experience.

Can't the web page embed the font?

You just don't have the required font.