well, if the same channel is used it overlaps, how closer they are the more it overlaps.
With other words this can indeed cause huge effects, usually you have auto channel, then it tries to get the best channel with the least noise.
To my experience a little noise is not a big deal, but i noticed problems start when something is super close, this router is especially vulnerable to direct interference when it is close, so if you have a second router this can be surely a issue.
^ it also happen if you test with a mobile phone very close to the router like 10 cm of it.
as for jamming... well i doubt it is jamming, jamming is often presented where the attacker sents a deauth packet in where the router deauths the device, a possibility can be a neighbour is using channel 1 and on the other side of his house channel 12 on 2.4ghz then he effectively has all channels disrupted.
Did some more testing yesterday, also in a other room to rule out any interferences.
The defective Flint 2 under <50Mbit load is actually jamming the whole WiFi network, specially the upstream of my other Flint 2 and a third WiFi AP, on totally different channels.
Channels (80MHz):
Second Flint 2: 116
Third AP: 48
Defective Flint 2: 100
The moment I switch the defective Flint 2 off or stop the WiFi load, the normal WiFi speed and latency of my second Flint and Third WP is restored.
I also ran the device without any WAN connected. The same problems by testing from device to router via iperf3.
