Dropping my friggin chain... and stumped.

Localized Acute Magnetic Interference?

So you're shifting to a smaller cog in the back, and the chain drops in front?  That does seem odd.

You might try adjusting the front derailleur high limit stop in a bit, so it just barely clears the chain when you're on the largest cog in back.

Don't know if it will make a difference, but you might also check front derailleur height.  It should clear the big ring by 1-2 mm, and maybe if it's a bit too high it might leave enough room for the chain to slip off?

Mind that I'm just speculating with both of those suggestions, I'm not a wrench and don't have any experience with the problem you're having.

If you try the same shift sequence but at a different location (on the trainer maybe) does it still happen?  What if you wait 30 seconds longer after coming down the hill or shift while on the hill?  Any difference?  I'm wondering if you're shifting while hitting a bad section of stutter bumps.

Grasping at straws:

Does this same thing happen if you are on the big chain ring or the smaller chainring?

Any flex in the frame?

Check the rear Derail, cable hookup - weird but on my rear derail you can pull the rear derail forward to the point of the chain being loose.  If the rear rear derail is pulling too far forward when you up shift the chain would drop.  Sounds pretty out there but it can't hurt.

Are you shifting more than one cog at a time when this happens?  Maybe too many too quickly?

Another shot in the dark--as you shift to smaller cogs, the rear derailleur cage has to spring back to take up the extra chain.  Maybe if it's not springing back enough (weak spring or maybe just a sticky spot in its travel), it's allowing slack in the chain which causes it to drop.