28 20 wrong grade display

Meaning erratic readings 

I did a 3 and a half hour ride with climbs  and the grade was looking reasonable. I think that on my previous ride maybe the atmospheric pressure was fluctuating. Any way the grade looks ok so it is not a firmware issue like in the past.

Well known fault of the current firmware. As workaround you can install CIQ-Datafields like "sensible grade". Just fixes the datafield, not the gradient readings in the climb pro screen etc.....

is BRM grade as well fixing it?

Try it, I just installed the one I mentioned.

I did a 100+ mile ride and the 1040 showed negative gradients for about 40 miles of the first 50.  However, the number of ft climbed was going up.  I dreaded turning around and coming back because I thought all of those negative gradients would be positive gradients on the way back. NOPE!  There were 40+ miles of negative gradients on the way back as well.  I finished the ride with 5000+ feet of elevation so the ride was not full of negative gradients.  This is bad when you clearly see and feel that you are riding uphill, but the gradient is negative on the garmin cpu.

Grades are still an erratic desaster with 28.33

And no word from Garmin-Clayton

Garmin-Blake

No recent Edge 1040 software updates include changes to the grade data field or elevation data. You can review all Edge software change logs in the Garmin forums: Edge 1040 Series Version 28.33

The Edge 1040 uses a barometric altimeter to measure elevation, and weather fluctuations produce the most significant changes in air pressure. Due to the many environmental factors present, it is unlikely that software is the cause of an elevation discrepancy.

For an elevation data concern as described here, we have specific troubleshooting steps and require additional activity data in order to determine where an issue is present.

This may include data we would not ask for publicly and we encourage all users to reach out to their local support office for the best solution: https://support.garmin.com/