Placing on face sometimes changes orientation  

Im afraid you will need to save your project (file>save project as) and then take the resulting .3mf file, zip it up and attach it to a post here for anyone to have any chance of working out whats going on.  Could be a bug or it could be the errors in your model (note the yellow warning triangle in the model tree list).

Auto-center parts enabled?

Any chance you have auto-center parts enabled in preferences?

Here is the file.

The caution is due to "errors" that were a result of having several pieces in the model (to make editing easier), but I had used Netfab to repair those in the original case.  I didn't do that to take the pictures, but the change in orientation changes in either case.

Thank you very much for the file.  On initial inspection I'd say its looking like a possible bug in the place to face function as there's really no need for it to rotate z axis too.

However its easy to work around.  Once you have flipped it over change the co-ordinate system in the tree view from global to local.  You will see that its changed Y 180 degrees and Z 73.27.  If you zero out the z angle it goes back to the 'proper' orientation.

RE: Placing on face sometimes changes orientation

@dik-harrison

The big "flat" bottom isn't really flat....

Rotate 90° in Y, or X and you will see this

edit: No caution in the file I downloaded

To late for edit....

Checked in Fusion 360, the part IS uneven, differs 0,003° to 0,009° when I measured between a few triangles.
I guess PS take the "biggest" flat area of triangles it could find and align it accordingly, maybe, I think....

Thanks to all...

I'll see if I can fix the model.

I don't think that is necessary on This particular part, it looks good in Prusaslicer and will probably print just fine.
As long as you don't try to print it the way I rotated it  😀 

The reason for the strange alignment was due to uneven flat face.

Out of curiosity, which program do you use

SketchUp Pro

I use SketchUp Pro 2018 (can't afford their subscription as a hobbyist user) mainly because I have used it from back when it first came out.  I've tried Fusion 360 and like it, but I'm much faster with SU.  Guess I need to stop basing my models on STLs of something close and draw from scratch so that these slight inconsistencies introduced from the STL won't be a problem.

Unfortunately I can't help you with SU.

Haven't used it for ages so I don't know how SU deals with STL's