Happens when you limit a site's canvas access to prevent fingerprinting, I believe.
Thank-you!
Since the expected permission request pop-up didn't show first time I visited the site, and the address-bar didn't highlight any blocking, and the settings dialog doesn't make it clear whether or not canvas is currently blocked (it has a question and two buttons, neither highlighted) I misinterpreted the default setting.
The 'streetview' canvas doesn't render correctly on Firefox 111.0.1 Linux amd64; instead it shows coloured vertical stripes that slowly 'scroll' from right to left.