Yes, it turns out that more than three weeks have passed since the firmware update, and during this time I did not find a problem with a drop in volume. That is, I can say that the firmware update completely solves this problem, it did not return after a while.
But now I'm facing another problem. I recently moved into a new apartment, and it so happened that due to the new environment and furniture, the standard meter USB cable from K7 became short for me. I connected the K7 through a meter USB extension cable and began to notice that sometimes the sound stutters, as if the signal disappears for half a second, it is perceived as a click by ear. At first I sinned on the USB extension cable, I thought it was of poor quality, but I connected my old FiiO K3 through it and did not find such a problem there. I recently bought an expensive 2m cable QED Reference High Resolution USB (
https://www.qed.co.uk/reference-high-resolution-usb.html) to make sure I didn't have a bad quality cable, but that didn't solve my problem either. Clicks (sound interruptions for half a second) with this cable became much less common, but still remained.
Possibly the problem could be poor power supply, I also ordered proprietary linear power supply JadeAudio/FiiO PL50, it should arrive in about a month, I'll check if this problem persists with it.
All of the above does not interfere with living and enjoying music. It's less of a problem than the volume drop, but curiosity takes over. But to be honest, I am very disappointed that from the very beginning it was not possible to simply buy the K7 and safely use it as it is out of the box. All these gestures to solve emerging problems are very tiring. Never had this problem with my previous K3. I had thoughts of upgrading to K9 or K9 Pro in the future, but there is no guarantee that problems will not arise with them during use.
