The problem here was auto-fill.
The normal hint is automatically hidden by my real email address (pixelated in the screenshot below):
[image: 1775270267359-6404e482-4e9b-4ed8-99ce-17d9e0b61814-image.jpeg]
Keyword in the screenshot above: whatever.
My email address is used for the Mastodon instance, but does not identify the instance. Whatever.
After clearing the auto-fill, the hint is visible:
[image: 1775270520828-85e70b11-6b73-4649-8a46-757476cfbf2c-image.jpeg]
Eg username@mastodon.social
The opening post is solved (tagged), I have flagged it for a move to the test area. Apologies for the noise.