OK – took a little time to investigate fully.
Colour profile makes no difference.
I discovered the way to cause it (and avoid it) – if the sub-sequence is a different frame rate (maybe resolution too, didn’t try that) to the one it is nested in and the still image is not scaled to fit exported media will freeze for the duration of the image.
If you scale it to fit it is fine. If the frame rate matches it works scaled or not.
Jules.