Dragging native app windows to extended display and back toggles between two widths

I use my MacBook Pro with two external HP displays, a main and an extended. The laptop itself is closed, so no third screen.

When I drag a Safari (or Mail, Notes, etc.) window from the main display to the extended one, it widens; when I drag it back to the main display, it contracts.

Both displays are set to 2560 x 1440. No window options like "Drag window to screen edges to tile" etc. are activated.

Not a major thing, but it does drive me a bit mad. (What really drives me mad is when all windows consolidate to the main screen after waking from sleep.)

Why is this happening? Sequoia 15.6.