@BurtNL This issue is caused by this commit https://github.com/joomla/joomla-cms/pull/17552/files#diff-43fc98bd347d2aeaa1b1628c8fb0829b

I am going to assume you have an override of the repeatable-table.php file. The mentioned commit created a B/C break.

The only solution there is, you have to update your override. Here is the code change from my extension:

Notice, we add the -1 everywhere because the new subforms require a unique number per instance of the form.

Here is the second change we had to do:

The tbody must be changed as well:

You must pass the unique ID as well to the $this->sublayout() function:

Finally you need to replace the script code with a template:

Finally you also need to update the section-by-fieldsets.php file with the following changes:

Notice we are also adding the -1 here.

Apologies for the screenprints but I think they are clearer than writing the text. You can contact me if you want the actual code.

@roland-d I was indeed using an overwrite of the repeatable-table.php and 2 other files: section.php and section-byfieldsets.php.
Those files had become outdated and were slightly different from the current files in /layouts/joomla/form/field/subform-files.
So I replaced the 3 outdated files with the new files as mentioned before. And the subforms are working properly again.

Thanks for your screenshots, which pointed me in the right direction, much appreciated!

Also, the subforms seems to work just fine, my issue was caused by some outdated overwrites.