Is this schemaorg specific or a global bug in Joomla when using subforms?

It's just bad markup in the xml

basicaly these are fieldsets not subforms

Just rechecked and rechecked.
The problem is 100% due to the schema plugin xml. There is no other xml form defintion in Joomla that has "nested" <form> elements.
In fact nested forms are very specifically mentioned

Warning: It's strictly forbidden to nest a form inside another form. Nesting can cause forms to behave unpredictably, so it is a bad idea
source MDN

(just because we call something a subform doesnt mean that it is)

If they would just render as a fieldset then it would be fine. but no matter what I have tried it doesnt render as a fieldset. I must be missing somethign obvious

This layout renders a "non multiple" subform: layouts/joomla/form/field/subform/default.php

yes i see that - which is wrong afaict and it should be a fieldset with a legend - see image above - but whenever I try to add it as a fieldest it doesnt render correctly. I must be missing something

The legend will not show up as you expected. It can be only within that layout.

On your screenshot it is a field label, that rendered in label layout.

In result can be:

<label></label> => from field label

<fieldset>            |
   <legend></legend>  | => from field input
</fieldset>           |

that is what is wrong and breaks accessibility and html standards

Following will not work?

<label for="blabla-input"></label> 

<fieldset id="blabla-input">            
   <legend aria-hidden></legend> 
</fieldset> 

If I right remember we use the same for checboxes/radio field fieldset.

Yes thats one option. The "difference" is that for checkboxes/radio we are creating a fieldset out of a single field. In this case we should be rendering a fieldest containing several fields

In theory it should be possible to do oposite: the label is hidden with hiddenLabel="true" in xml, and the legend visible.

it shouldnt be necessary. we already render multiple fieldsets elsewhere

https://accessibility.blog.gov.uk/2016/07/22/using-the-fieldset-and-legend-elements/