A Google search results in https://www.amigasys.net/

This should be largely fixed in the ext update.

Some more testing between your test page and the other more graphic one I've found reveals that this is down to passing through the native keycode as is.

I had a note in the code that this may be the wrong thing to do :-)

The curious thing is that the unmodified raw key worked up key up (on the key test ru page at least) I'm now thinking tt that i because it was out of range, so the code was falling back to the interpreted value, which passed separately.

The irony here  is that taws is probably converting this keycode back to an amiga rawkey code when it already is.

I've tried converting to linux input code, as it's running on linux backend CEF might expect that, and I have a convenient premade table for that for the linux builds of my MKShare tool,  but it appears not. That gives worse results!

I solved it in the end but it was a journey.
I used another card - Radeon RX 560 and that one works.
It has DVI, HDMI and DP, it doesn’t behave well over DVI but HDMI works fine at full 1920x1200/60.
Not sure what’s going on with the RX 550 that only works in boot menu and kickstart.
I also tried a RadeonHD 5450 and that one works in boot, kick and even shows the „AmigaOS 4.1” splashscreen with some artifacts, then the system loads and the screen is a total mess…

I set up Amiga OS 3.2 from Emu68 imager (without newicons).  I found I could not mount ADFs. 

I updated to 3.2.3, ensured DefIcons is running, but still, when I double click an ADF it just wants to execute it and fails. Icon looks like a hammer.

I suspected maybe DefIcons needed newicons to function properly, but I just don't know.

Could someone please help

Thank you!

Sounds like you have overwritten the deficons with newer ones without the correct tooltypes defined.
Try copying back the original deficons from the AmigaOS3.2 CD. Then go from there.