And I select frame 1 using the select tool and I don’t select frame 25. I then press the button the frames 1 to 24 are displayed as see through and the bounding lines are shown in light blue.
From frame 25 to frame 40 the frames are displayed with solid colour and the blue lines disappear.
The .GIF file Layers-2.gif show this effect!
I assumed wrongly that pressing or exporting the frames would automatically reset this selection.
If before I press play, I deselect the frames the problem vanishes. Or I have to accept the see through and blue lines and a transient effect, which if I deselect the frames will not exist in the exported files.
What you encountered is one of many long-standing bugs with the vector layer in that the selection highlighting UI is rendered even during the exporting process. This is unfortunately not a trivial fix and until it is fixed you will have to remember to deselect any vectors before exporting.