You have to be pretty careful, about the "server selection", if you want an even close comparison and even then there are other "factors", such as where the server is (other traffic), how much load it has (other people testing) and more, I usually run a few times on the same server, if I want a more "normalized depiction" of the results.
Or I run a LAN test, to my server, that's close to 100% reliable, handier, but not as "real world"
I'd try resetting the network settings, which nukes all your cached WiFi values, including DNS and other artifacts that might be giving you grief.
I don't see this *exact* issue, but have a couple of other WiFi, a sound problem, and one charging issue with my OP6 (just the biggies, lots of small stuff), it's not exactly the "most stable" phone I've had; pretty close to as bad as Win10 phone builds were, at the lowest, or close.
I feel like I should've stuck with my 5T, but my wife's 6P was dying, she got the 5T, and I got, well...
My first OP6 is already winging its way back, it had some USB port issues: insanely varying charging values, you could almost throw a dart at the wall, as to whether or not it would charge (at all), or how fast (amps), or ...
I've now sent OP 3 sets of full Android logs, per request from OP, for three big repro issues in a couple of weeks. In my 5T timeframe, I had one of these (that was serious enough to warrant a bug, and repro/logs to OP).