WELP just happened to me! Once during a post on this account, and actually just again when I was responding to this thread.

Are you using the same browser on all devices, if so are they synced? No, I use Chrome on my laptop and Safari on my phone
Does it always happen on the same wifi network? | Only happened once, but I have been on my home wifi network with both times
Is it refreshing the entire page or just the text box? | Just the text box - part of the message disappears
Does it happen on all accounts or does it seem to be only some e.g. maybe new accounts? | Only has happened to this account so far
Do your accounts have the default text editor set to source mode? | Yes
Does it happen immediately after switching accounts? | No

One thing I did notice is that both times it happened, it happened AFTER I'd copy + pasted something into the box. For the post, I'd written most of it in Notion, then c+p'd over to Charming. Then I added the post tags and bold tags for text, then wanted to add another sentence, so I was typing in the middle of the c+p'd field.

For this response, I'd just c+p'd the questions, and started adding the message above the c+p'd field.

I'm DMing you the screenshot I have of the errors that came up on my end when I inspected!