Unfortunately, at this stage, understanding many elements of this bug are beyond my capabilities, but I can confirm #3277784 caused a number of sections, on a site I manage, fail to render after upgrading to 9.5.9. Issue summary above seems to align with the problem I've encountered. Applying the reversion patch in #2 fixes all issues and I'll be using that so I can take the site to 9.5.9 for now.
Using patch #5 the missing sections of the site are rendering again, but the patch creates new problems with the site breadcrumbs. The site has both current_page_crumb and menu_breadcrumb installed and with patch #5 applied (no issues with #2 reversion applied) I'm getting duplication on the last breadcrumb entry and only on term pages. Sorry, I have not been able to isolate why.
Given upgrading from 9.5.8 (or prior) to 9.5.9 can result in parts of a page no longer rendering (causing a blocker to upgrade for some) is Normal Priority flag sufficient for this issue?