I think I have seen the same in my return to running (at age 65).
As I ramped up, I was running timed intervals like run3/walk1 and run5/walk1 for 5k, and then zone 4 intervals (run till hear zone 5, walk till hear zone 3) for 5k. I was averaging 39-40 minute 5k times. (VO2 around 22.)
Now I am up to running for 35 minutes, walking a bit, then run2 walk1 till done the 5k. My times are actually worse, 42-43 minutes. I am running more but running slower for a calculated VO2max of 20.
The other factor that might be influencing this a little, is the temperature here in south Florida has increased from 70s when I started five months ago, to nearly 90 yesterday. I really need to get out earlier in the day, instead of sitting at the breakfast table reading the news of everyone else’s lives.
So my suggestion is to mix some slow runs with some interval runs and some whatever runs, and don’t worry so much.