They were both set to “yes”, which I actually don’t want that behaviour, so have changed to “no” - still no difference. It’s looping through the Playa, but not able to output the values within.

3 different attempts to get it to display…

{blocks}
{block status="not closed"}
<br>Playa in Matrix<br>
child={child_id} 
title={title} 
{entry_id}
#{block-layout}
#{position}
{/block}<br>

{block var_prefix="playafield" status="not closed"}
<br>Playa (with prefix) in Matrix<br>
child={playafield:child_id} 
title={playafield:title} 
#{playafield:entry_id}
#{playafield:block-layout}
#{playafield:position}
{/block}<br>

{exp:playa:children field="block" row_id="{row_id}" status="not closed"}
<br>Playa (full tag) in Matrix<br>
child={child_id} 
title={title} 
#{entry_id}
#{block-layout}
#{position}
{/exp:playa:children}<br>

                    {if "{position}" == "sidebar"}
                        {block limit="1"}{block:entry_id}#{block-layout}{embed=block-embeds/{block-layout} position="sidebar" entry_id="{entry_id}" parent="{parent:entry_id}"}{/block}
                    {/if}
                {/blocks}

blocks being the Matrix field, block being the matrix cell containing the Playa. But maybe I should raise with Pixel and Tonic…?

For anybody else’s reference: https://boldminded.com/add-ons/publisher/persistence