As far as I know, if you haven't used those room mailboxes, they won't appear in location suggestions.
You could get the availability status of the room location you want to use by typing the first two to three letters of it.
Here are more details on location suggestions, hope to help you!
In order to troubleshoot further, does this occurs on the outlook web app too? Was it working before? Do we have users this is working for? What is the difference between the working users and the none working users? Confirm if outlook is updated too.
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