I think I've found the root of the problem. I removed the camera from the scope and installed the supplied fisheye lens designed for use as a standalone CCTV camera. This gave an image, at the specified resolution, but only of the top-left part of the frame.
I started from 960x540 - quarter of the advertised maximum - and worked up from there through the 16:9 ratio. As I increased the resolution, the visible portion increased from the top-left quarter to showing slightly more of the bottom and right. Eventually, at 1536x864, the same behaviour as for 1920x1080 occurred and no images were transferred to the client.
I then tried the same thing using CCDciel connected to INDI and saw exactly the same pattern repeat. However, using the supplied AltairCapture software and a direct USB connection gave an expected image of 1920x1080 resolution and showing the full frame.

As this problem is clearly outside Ekos, I've started a new topic under CCDs and DSLRs, linked here: https://indilib.org/forum/ccds-dslrs/6189-altair-gpcam-290m-showing-cropped-frame.html

Thanks again for all your efforts in helping me!

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