Quite simply, when charging on a stand, the auto-rotate of the charging screen fails and defaults to sideways.
When the sleep routine is active, it rotates to face physically up for a few seconds, then flips back to the default position as if it's being worn (sideways).
Since the sleep routine displays a pop-up on the screen when the watch is woken up, I'm guessing that's interfering with the auto-rotate on the charging screen.
This isn't really a help request and more of a bug report. I couldn't find anywhere else to report this.
Turning the sleep routine off, removing my watch from the charger, and putting it back on fixes it. Enabling the routine on my phone will make the watch flip the wrong way after a few seconds.
