You need to look at the actual data rather than a summary to see the GPS health.
Click in the Download CSV on the Phantomhelp summary and go through the spreadsheet of data, looking at Column T "GPS Level"
That shows the drone having GPS Level1/5 for the first 40 seconds and dropping in and out of 1/5 many times during the flight.
It only had good GPS for the period 5:23-8:51 when it was above 400 ft and seeing more sky than it could down in the valley floor.
The period 19:16.6-19:20.7 is the one of concern.
Although the GPS receiver was getting signal from 8 sats but the flight controller had no confidence in the position data due to a poor satellite spread, multi-path issues or other factors.
Although it had not officially dropped into atti mode, the flight controller was aware of errors in the GPS data and had the lowest confidence level in the data, as it had for several other parts of the flight.
Flying in a poor GPS reception area as you were, it's a good idea to look at the number of bars shown next to the satellite icon on your display for an idea of how reliable GPS is.