Speed drop on Fritzbox 7530ax

Ok a slight weird one, I have a 900meg FTTP from City Fibre connected to a 7530ax.
I'm experiencing a strange one (confirmed by Zen today) What is happening is that when the router has been rebooted and within the first 3-4 mins all speeds are fine 900 up and down (all tests via Ethernet) After this. time frame has expired the speed drops on download to around 520meg but only on Mac OS Tahoe, Windows runs at full speed 900 up and down. Prior to Fritz firmware 8.20 everything was fine on running anything Tahoe or Sequoia, Sequoia works fine under Fritz firmware 8.2.
My neighbour is experiencing the same issue and I've also tried replacement 7530ax's which exhibit the same same result.
The Zen engineer tried an Eero 7 and that behaved perfectly apart from it being from Amazon and stupidly expensive. In the end it had the engineer scratching his head.
AVM are sticking there head pretty much in the sand saying nothing wrong with our kit (there wasn't till you updated the firmware)
Now for the crazy part I do know someone with a Mac mini M2 pro and Fritzbox 7530ax but on open reach and his behaves ok.
Tried the usual culprits of EEE, speed caps, ethernet cables, reboots of ONT's, fresh factory reboots of the 7530ax, reinstall of firmware, wipes of Macs and installs of clean OS.

So I wonder if anybody else has experienced this crazy with Mac Tahoe and 7530ax firmware 8.2 and Ethernet (although I'm actually questioning wifi also because its always lower than upload by about 60-70 percent)

Zen asked me to test Wifi and It looks like there is a cap on the wifi also, if you switch out to another fritzbox running earlier firmware everything is fine 900meg on Wifi and lan.

The question is iit specific to City Fibre and why only Macs running Tahoe although my friend on Openreach is questioning some of his results / tests

I am having exactly the same problem !
7530 ax and zen 900 full fibre
The only way I can sort it is by binning the plists and then it lasts ok for a few hours /days
Not technically competent to help more but has only happened with Tahoe

UPDATE
Spoke to Zen who are aware of the issue and have raised it with AVM (fritzbox manufacturers)

At least I now know I'm not going mad

Thanks for reporting this, I assumed that congestion was the reason my (500/500) was only getting 260 down on my 7530ax router, swapped back to a 7590 on ver 8:20 and back to approaching 500 down - happy days!

I'm guessing Steve that your running Mac Tahoe, interesting about seeing just above half your full speed, as that's what I'm seeing.