I was the one digging the hole instead of pushing the buttons for that shot, but I think I figured out what happened.
We were just out of range for the UHF radio with the internal antenna, and my trusty assistant must have pressed the Save for PP button. She said the UHF box was red and counting. I switched to external antenna for the next shot when I noticed it was having trouble staying fixed.
It was out in the open. A fence post shot that didn't need much precision, but it does have good agreement with PPK.
Is there a reason that it is labeled RTK? Sounds like it should be just DGPS or CDF. Does the RTK label just indicate that there is a simultaneous base file that can be processed to give a PPK solution? If so, maybe there is a clearer way to indicate that.