Massive Packet Loss/TCP drops with NWA220AX-6E under FW 7.00

For several weeks the central firewall log of the core router had been showing many TCP connection drops, which indicated delayed TCP packets.

These only occurred in connections via the Zyxel NWA220AX-6E. On closer inspection, I found that the uplink speed to the connected 2.5G uplink port sometimes dropped to only 100M (displayed as such in both the WebUI of the AP and the switch). With several new cable connections, this occurred from time to time or remained at 2.5G.

In addition, editing the settings in standalone mode via the AP's WebUI had also become extremely slow, and there were frequent disconnections (with an alleged "3-minute URL timeout").

The AP was then replaced by support. I used the replacement device (with different cables and completely replaced upstream hardware) for several weeks afterward. Unfortunately, the symptoms did not change. Another temporarily installed wireless router on the same switch worked perfectly.

It took me a while but then I remembered that I had upgraded the firmware of the NWA220AX-6E in the summer to the new 7.00 branch (all the way to the newest V7.00(ACCO.2)). Today I downgraded to V6.60(1) / 2023-06-26 00:36:51 (which I thankfully had saved before, because it cannot be downloaded from the support site anymore). All issues instantly vanished. I therefore suspect that all of the V7.00 firmware packages (I started with the earliest V7.00(ACCO.1)) are somehow broken: they did not work on my initial device and also the completely new replacement device (which was delivered with the V7.00(ACCO.2)).

Is this a known issue?