Partial Solution.
By removing the vertical scroll bar from one subform and using navigation buttons in its place stops any flickering anywhere when using the scroll bar on the second sub form.
This would suggest that somehow Microsoft have got a problem in the design of their vertical scroll bars if using one causes a flicker in the other.

Without seeing it in action in your database I can't be sure, however, I do get a slight flicker in the same manner using even a single continuous form. If I use the slider, the detail rows move smoothly up and down but clicking the arrows at the top and bottom create a slight flicker in Access itself.

Only a guess based upon my experiences of getting rid of flickers: The arrows are tied to the window and the slider is tied to the section. Even though it appears as a single control, the arrows seem to fire a different mechanism than the slider. Like the arrows are redrawing the page and the slider is only redrawing the section. As a side note: In my case, if I actually grab the part of the slider and move it, no flicker, but if I click the slider bar outside of the active slider section, it gives the same flicker as it is moving several "pages" instead of pulling up an already queued next row.

The only way I can think of to get around this effect would be to turn off scrollbars completely and create a custom solution with controls and vba. I've never done it specifically but it would involve some of the Recordset and RecordsetClone techniques Richard has shown such as in the Fitness series where he shows his bounce function to move through a continuous form with little to no flicker.

You're definitely not imagining it. There have been other reports of flickering and redraw problems with continuous forms in Access, including cases where the behavior changes depending on how you scroll. In at least one publicly reported case, the problem could even be reproduced using Microsoft's Northwind Developer Edition, so it's not necessarily something wrong with your database or form design.

Microsoft has also acknowledged and fixed a more recent, separate continuous-form redraw issue. I don't think that particular bug is the same one you're seeing, especially since you've reproduced yours in both Access 2010 and Access 2024, but it does reinforce that Access has had issues in this general area of form painting and scrolling.

Based on your testing, I'd say your workaround is probably the most practical solution for now. If removing one of the native vertical scrollbars and replacing it with navigation buttons eliminates the flicker, I'd go with that. I wouldn't spend much more time chasing Echo, Painting, Tab Stops, or record source changes if you've already eliminated those possibilities.

For what it's worth, Microsoft is aware of continuous-form redraw and flickering issues in Access and has recently fixed at least one related problem. I can't say that your exact scrollbar issue is the same bug, but this general class of problem is definitely on their radar.