Thanks for responding Bojan. It’s bizarre, yeah. The post types are default, as well as 3 different custom post types. The rule to display the custom sidebar is just assigning it on a per-post manner, from within the edit post at wp-admin/
What I am going to do (because I want to double-confirm this) is re-assign the custom sidebars for all these posts at the same time, and wait to see if it happens again.