I discovered, that my AC-7260 completely stops dropping packets while fully loaded, e.g. while run iPerf benchmark in infinite loop.
I simultaneously benchmarked the network and pinged the router for a really long time (~3 hours) without any lost packets. Remember that without stressing I had a lost packet at a rate of approximately once in 10 seconds.
Reply from 192.168.137.1: bytes=32 time=26ms TTL=128
Reply from 192.168.137.1: bytes=32 time=28ms TTL=128
Reply from 192.168.137.1: bytes=32 time=31ms TTL=128
Reply from 192.168.137.1: bytes=32 time=23ms TTL=128
Reply from 192.168.137.1: bytes=32 time=26ms TTL=128
Reply from 192.168.137.1: bytes=32 time=30ms TTL=128
Reply from 192.168.137.1: bytes=32 time=30ms TTL=128
Ping statistics for 192.168.137.1:
Packets: Sent = 9668, Received = 9668, Lost = 0 (0% loss),
Approximate round trip times in milli-seconds:
Minimum = 0ms, Maximum = 157ms, Average = 27ms
So, this kind of continuous stressing is definitely impractical solution for the issue, but it makes me wonder why it works this way? Why everything works at full power only? Is this kind of a power saving failure, WiFi driver is programmed to save power while at partial load or idling, and now that fails and leads to lost packets? I already have set the adapter to maximum performance in power settings.