earliest release where this has been observed

you're all but one ;-)

What's actually going on is a little different than it first appears. If you move the fingering by either dragging or moving directly with the mouse, this issue doesn't happen. It only happens when using the Inspector spin box, and it happens because there is a conflict between the numeric value being set for offset and the "minimum distance" required in order for that offset to actually apply the way you want. It's complicated to explain, but this is another aspect of the same basic issue as #293469: Fingering repositions unexpectedly when multiple elements set to same offset using Inspector as #291990: Recently adjusted fingering can shift when changing style setting. I'm leaving this open because in implementing a fix, we will want to make sure we cover all these cases.

For the record, the unchecking of autoplace on the slur isn't specifically relevant; any operation that triggers a relayout of that system will trigger it. The issue is that the position set by the Inspector is not reflecting the true position that should be reflected by the numeric setting. In other words, the position afterwards is actually correct given the setting in the Inspector

I think the specifics of this case probably apply only to fingering, and as such, I would recommend making position adjustments directly with the cursor keys rather than the Inspector for fingering.

Implementation note: I think perhaps the "rebasing" calculation done to compute the correct offset to apply after beginning a manual adjustment is not right for fingering. Probably the offset adjustment does not actually actually need to happen in this case, or at least not the one that happens within rebaseMinDistance().