autofill fails on webssites - it either fills in nothing or wrong things

Sorry this is going to be long and detailed (I've done a lot of beta testing in the past) - let me know if there is a better way to provide this info!

So if I go to that page you provided I get the form in screenshot

It is in English (my browser language is English) and my OS is English (bilingual is a challenge to computers clearly).
Nothing is filled in and no 1P fill options are visible
If I click in the fields nothing pops up to say 1P can fill it
If I use the plugin from my browser toolbar it fills all in perfectly
Right clicking any field doesn't offer any autofill options from 1P is that not a feature (lastpass has it)?

I think it would be hard for you to get the form I originally showed, here is a form you can access:

https://www.marktplaats.nl/account/createAccount.html?target=%2Fmy-account%2Fsell%2Findex.html
This shows some good behaviour but not as good as it could be

there are no 1P symbols to help you know it can be autofilled however clicking on a field shows that 1P can make a suggestion - I think that 1P should at least make it clear that it can fill something.

When you click in the 1st field it offers 3 identities - I think that is OK
However in the email field it still offers my potential username as well as my 2 email addresses - that's not quite right

It offers to make a password correctly - that's good

Here is a different form

https://www.praxis.nl/voordemakers/register?redirectUrl=%2F

this is in NL and If I click in a field no 1P symbols in the fields tell you that it can fill anything

However it will fill from the toolbar plugin button if I press autofill next to the identity I want to use

It does show up a few issues I'd like to get into but not sure this is the place (it doesn't fill some things and that is because there is no clear way I could see to add them to the Identity (to do with the way NL addresses are structured).
It misses some things out - the non-blue fields
It fails to fill my birthday correctly (gets the day right but not the rest)

Any other things I can test?

I'm an IT person and I can see that this is hard to diagnose - I don't mind if we leave this thread open until I find another case like the first one where it clearly won't fill. Is that OK?

Should I start a separate thread about the partial filling and the UI aspects I brought up in this one?