Activity screen alternates black/white during a run?

So- I'm on a run today, and the screen background turns from black on white, to white on black.  it would do it for maybe 30 seconds at a time, every half mile or so.  

I do have the AutoClimb feature set to ON and the Invert mode on... which would normally be the cause- except I was running on a flat river path today.

in 10 miles there was less than 200 feet of climbing.  My AutoClimb is set to 1800ft/hr- and there's no way I hit that.  It didn't even flip when I raced a half marathon on a hilly course Sunday that had nearly 900ft of climbing in it.

1. what else could cause temporary screen flipping to white on black?

2. I thought it was easier to read in sunlight.  is there any way to make that the default?

thanks in advance

Fenix 6 pro with beta 4.62