Additional code appearing in HTML view of a section

https://aireboroughhistoricalsociety.org.uk/jack-test-2/

This website has thousands of pages. I started the first few pages off and then trained Jack up to add more. Over the past 18-months Jack has added lots of pages.
Jack has been adding the content and that has been going fine until a few days ago (25th August) when he told me that the pages were putting in extra code.

When he puts a picture in

Instead of seeing

<figure class="wp-block-image size-full">
<img src="https://aireboroughhistoricalsociety.org.uk/wp-content/uploads/2022/08/3945-1JDenison.jpg" alt="3945-1 John Denison 1939-1945" />
</figure>

Which he then edits to create

<figure class="wp-block-image size-full">

<a href="https://aireboroughhistoricalsociety.org.uk/wp-content/uploads/2022/08/3945-1JDenison.jpg" rel="lightbox">

<img src="https://aireboroughhistoricalsociety.org.uk/wp-content/uploads/2022/08/3945-1JDenison.jpg" 
alt="3945-1 John Denison 1939-1945" class="wp-image-32451"/>

</a>

</figure>

Which then creates a popup window and if there are more than one picture on the page you can then scroll through the pages.

Jack gets this code

<div class="wp-block-uagb-image uagb-block-5b4a2cdb 
wp-block-uagb-image--layout-default 
wp-block-uagb-image--effect-static 
wp-block-uagb-image--align-none">

<figure class="wp-block-uagb-image__figure">

<img srcset="https://aireboroughhistoricalsociety.org.uk/wp-content/uploads/2022/08/3945-3JDenison.jpg 1024w" 
src="https://aireboroughhistoricalsociety.org.uk/wp-content/uploads/2022/08/3945-3JDenison.jpg" 
alt=""  class="uag-image-32453" title=""/>

</figure>

</div>

Which is not all separated out and makes it a bit confusing to add the code to create the popup pictures.

In our test today.
When I create the picture I get the expected result but on Jack’s computer he gets the additional code when we are working on the same page.

I am hoping that this must be something on Jack’s browser / settings where he has accidentally turned on which is creating the additional code to appear.

Any thoughts most welcome.

kind regards

Charles