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Hello Rob,
Thank you for posting in Microsoft Community and sharing your concern.
I would like to confirm the followings:
- What's the current number of list items for affected list? Just to ensure it hasn't exceeded the view threshold - 5000 items.
- Does this affected list inherit permission from site collection or it has unique permission?
- You can still manage list item via: select item > manage access > advanced setting to restrict access, am I right?
After you confirm all above, I would recommend you log a ticket from Microsoft admin center to online support team to involve the backend team to check further.
Best regards,
Nero-TR - MSFT | Microsoft Community Support Specialist