Shadow topic read/unread indicator not working

[Working theory, for tracking purposes, see prior related discussion]

Device: PC
Browser: Firefox

Steps to reproduce:
- Move topic from forum A to forum B
- Leave shadow topic in place in forum A
- Await new posts in topic
- Navigate to forum A
- Observe read/unread indicator cross for shadow topic in forum overview page

Anything else to add:
E.g. price-check/ring-appraisal-t1617749.html viewed via forums/
Indicator seems to always indicate 'read' (gray cross) for shadow topics, even when there are new/unread posts inside or the entire topic is new/unread to the observer.