unwanted rotation issue (reset?) of StaticMeshActors

I’ve imported a STEP (also IGES) file via Datasmith into UE 4.23, which gives me the following structure in the World Outliner:

parent (DatasmithSceneActor)
→ child_1 (Actor)
→ child_2 (StaticMeshActor)(Instance)
… with some deeper nesting also occuring.

Directly after import everything is fine and looks as expected.

Then after some time (sometimes just after selecting random objects, starting to build…) some child_2 elements jump to a weird rotation. This is not undoable and not present in the undo history. This doesn’t happen to all affected children at once, but their number seems to increase over time (or action???). It also only happens once to each affected child like some sort of reset to an initial state of rotation.

Children under following circumstances are affected (as far as I have managed to observe):

The rotation of the parent and the child_1 elements is left untouched as imported:
X:0,0° Y:0,0° Z:0,0°

The rotation of those child_2 elements affected have some negativ rotation after import, e.g.:
X:-147,94° Y:0,0° Z:90,0°

For these elements I also experience a weird behaviour which I can not reproduce by manually attaching element stacks that I create from Unreal Basic Objects (inside the same project/level/scene):
If I drag the slider for e.g. X and release it, all three input sliders jump to a different value, as they were before, while the object rotates visually as desired.
Objects added to the scene manually by me just behave like expected: dragging X only affects the X-rotation and Y and Z stay as they are.

Is this a bug or am I missing something?

One way to reproduce this is by saving using the Save Current button and reopening the level. Sort of as described here:

Then all affected elements are rotated.

It doesn’t seem to apply for all children with negative rotation… Scale for all elements is left untouched at X:1.0 Y:1.0 Z:1.0.


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