Unexpected / Invalid block content since 6.1.1

If I understand correctly, in your block editor texts prior to version 6.1, you have customized the HTML code generated by the list editor? So, for example, you manually customized the content in the code editor like this:

<!-- wp:list -->
<ul><li class="firstclass">first</li><li class="secondclass">second</li></ul>
<!-- /wp:list -->

Is this correct?

In wordpress 6.1, the list block has been revised. The individual list items are now themselves blocks that are inside a bypassed list block. This allows you to give list items their own classes via the editor interface under Advanced. In the code editor it looks like this:

<!-- wp:list -->
<ul><!-- wp:list-item {"className":"firstclass"} -->
<li class="firstclass">first</li>
<!-- /wp:list-item -->

<!-- wp:list-item {"className":"secondclass"} -->
<li class="secondclass">second</li>
<!-- /wp:list-item --></ul>
<!-- /wp:list -->

My guess now is that your customizations to the HTML code are lost in 6.1 precisely because of this customization. This is also why you get this message.

My recommendation would be that you report this to the Gutenberg team: https://github.com/WordPress/gutenberg/issues/new/choose

Many thanks for taking the time to give me such a detailed response @threadi. Your understanding of the situation is correct; I didn’t know that list items could now be given classes via the editor without going into HTML mode, and my only takeaway from a recent upgrade was that it now takes me one extra click to change the default UL (which I hardly ever use) to an OL, which I use all the time. But I digress. ??

With regards to HTML customisation being lost, there are two sides to this, one positive and one negative:

(+) classes which were added to LI using HTML mode in old posts are picked up correctly in the new editor at LI level, and I’m not inadvertently scrubbing HTML customisations by making other edits to those old posts. There’s always that danger when parsing existing content to support a new UI feature, so we’re all good here.

(-) when I add a class to a newly created list using the editor it still creates an error as soon as I switch to focus from the LI to the parent OL and enter HTML mode. To be clear I’m not actually changing anything in the HTML editor, it’s just turning its nose up at the class name that was added with the UI focussed on the list item.

Can you do me a favour and briefly confirm the behaviour I’m seeing please @threadi? Create a list with a couple of list items, add a class name to one of those items, then switch to the parent list and toggle in and out of HTML mode without changing anything.