🇺🇸United States @tlilleberg

I created a blank Drupal install, enabled section_library and confirmed it was working as intended.

Then I applied the patch. Afterwards the sections I added to the library were still displaying correctly in the admin page on in LB. I was able to still apply section libraries successfully.

I then added a simple text field to Library Template entities and was able to edit and save data into the new field.

section_library_fieldable-3442106-9.patch works with no issues.

I was having a similar issue but it turned out not to be an issue with core. My enormous header was edge-cache-tags from the akamai module. Disabling the purge and akamai modules on local took care of it.