OK, this bug is INCREDIBLY specific, but unfortunately is completely breaking my project. Example PDF below and INDD file in Google Drive folder here:
https://drive.google.com/drive/folders/1teEtxzm5lT5PnwsZ6BKToDpSXgAx4D9d?usp=sharing

I'm trying to make a multilingual instructions manual (A4, facing pages, 3mm bleed on all sides) with tabs on the page edges for each language. I have one master page "A-Gabarit" which has all the general layout elements and all the language tabs grayed out, and for each language, I have a master page based on A where I override the one tab for that language to color it black (so the EN master page is based on A and has the EN tab overridden, the FR master page is also based on A and has the FR tab overridden and so on)

What's happening is that depending on how the spread's pages have their master page assigned, some objects dissapear when they shouldn't:
- If both the left and right pages have A master, everything displays fine
- If the left page has A master, and the right has EN master, then anything from A that was overridden in the EN master dissappears from the left page, the rest remains.
- If the left page has EN master, and the right has A master, then everything works fine.
- If both pages start with A master, and any other master page is applied to the right page, objects overridden in the right page's template will dissappear from the left, even though they were there right before. They reappear if the right page's template is switched back to A again.
- Finally, and worst part: if the left page has EN master, and the right page has FR master, objects from common parent A master that were not overridden in EN, but were overridden in FR, will dissappear from the left page, so it doesn't actually matter that the left page has the parent master page or another child master based on the same parent, just that it shares elements from the same parent master page as the right page

The issue only ever appears on the left page

PDF output is affected, it's not just a display issue.

My workaround, so far, is to duplicate the colored tabs in each child variant on top of the existing one instead of trying to override them, so that the original is preserved and doesn't dissappear on the left page, or alternately, rebuild all master pages with no "based on X" relation and no overrides, defeating the purpose of having master pages based on other master pages (ouch)

I'm guessing that when applying master pages on the right page, InDesign is handling overrides on a spread-level instead of a page level, which is problematic when both pages of a spread are using different master pages based on the same parent master page (this has never been an issue for me up until now). Expected behaviour would be for each page to show their own master page content in any context without interference from the facing page.