Let me start out by saying... it indeed was a couple abnormalities regarding how the drone obeyed commands for a descent, and it hadn't anything to do with control signal glitches... and it wasn't any faulty drone that caused it either.
The cause was likely wind related, together with how the firmware prioritized between holding the heading speed up, preventing side drift & at the same time slowing down the motor RPM to descend.
This unresponsiveness to descend only occurred when you commanded max heading speed with a full elevator stick input, together with a negative throttle input for a descent.
The windspeed from one of the occasions where this happened is shown below... the wind was coming in from south & the flight direction was in an nearly exact easterly direction, pretty much a precise side wind.
View attachment 173760
During this section the drone struggled to prevent a northerly drift & at the same time obey your command for max heading speed. The roll & pitch angle was nearly the same, about 20 degrees, this together made up a total tilt angle of 30 degrees... which is max for the mode you flew with.
The drone mostly managed to prevent a northerly drift & at the same time achieve those 12m/s (27mph) that is max for the mode. But here you also commanded max descend speed with a full negative throttle input... nothing happened here at all, the vertical speed stayed at 0m/s. So all became a prioritization game...
-allowing a descent without scarifying the heading speed = drone starts to drift north
-allowing a descent & preventing a northerly drift = the heading speed disappears
-allowing the commanded heading speed & preventing side drift = the motor RPM's can't be lowered to make descending possible.
DJI went with the last combination as a highest priority...
As soon as you let off from the elevator stick for full heading speed completely... other opportunities opened, which allowed the drone to direct the tilt angle directly into roll & by that prevent the drift easier & at the same time lower the RPM for a descent.
Here's the original .TXT log... upload it to either PhantomHelp or Airdata yourself for a quick overview.