This came from the ARRL Contest Update 6/12/2019:
Turn off the Radio AGC to Increase Digital Decoding Accuracy
From Ed, W0YK's presentation at 2019 Contest University: Decoders for digital modes tend to work better when the radio AGC is turned off. If you can't turn off AGC, then set it to SLOW. "Using a fast AGC increases the decode error rate in modern software decoders." Also, make sure you are NOT using any twin-peak or other specialized filtering provided by your rig manufacturer. Chances are, the software decoder will work better without it.