When browsing to the "source" and "destination" pages of a given migration, one gets the following notice (either directly on the page or in the logs) : "Notice: Undefined index: #description_display in template_preprocess_fieldset() (line 219 of core/includes/form.inc)" (also on line 220). Apparently, we only need to add this key to the fieldset definition.
This is of course a minor bug, but was already spotted a few years ago → by mikeryan.
Fixed
6.0
Code
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It's likely this issue predates Contrib.social: some issue and comment data are missing.