unable to save custom layout on Plone6 classic

In a new Plone 6 Classic site, I can install and enable in the control panel, restart the instance and create Documents with Mosaic layouts. So far, so good. Then, I want to customize my layout and save it... The save button in the modal window does nothing.

In contrast with e.g. the contact form, the Mosaic save button does not have any event actions associated with it. I am at a loss on how to proceed...

Is this with Barceloneta? There has been issues with other themes in 5.x. I think the workaround was to 1) Save a layout in Barceloneta, 2) Switch theme.

as vanilla as it gets: create site, install add-on, restart, create content, customize layout, fail...

I can confirm this in the Plone core development buildout with a checkout of Mosaic.
Can you open an issue in the tracker? Issues · plone/ · GitHub

Thank you for confirming, I will open an issue