Since yesterday, hitting Enter/Return does make the cursor go down a line
 

This is happening both on an iPad and on an iMac...

When I type in an email and hit Enter/Return more than once (say, to separate paragraphs), sometimes I see the cursor jump down a line then immediately jump back up to the start of the line immediately below the last typed line. The carriage returns are there, but the cursor doesn’t stay where it should; it jumps back up to the line right below the last typed line.

This has been happening for the last day or two.

I can confirm. I see similar strange behavior using the iPhone FastMail app and the iPhone safari browser through Fastmail's webmail. I don't see any problems using my firefox web browser on my computer.

I am also having this problem, or something very like it.
By coincidence, I bought a new computer yesterday, and I had put my difficulties editing e-mails down to my unfamiliarity with its operation (the keys are laid out a bit differently). But it definitely isn't that.
It's something of a pain, to be sure.
Has anyone raised a helpdesk ticket?

I have raised a ticket, yesterday. Haven't heard back yet.