Older Emails not showing in group mailbox

Are you using cached mode? And what's the value selected for the "cache slider"? The reason I'm asking this is because in newer Outlook versions, the same settings apply to both the primary and any additional mailboxes, i.e. automapped shared ones. So if the "main" mailbox is configured to only show 1 year worth of emails, the same will apply tot he shared ones. Here's an article with more details: https://learn.microsoft.com/en-us/microsoft-365-apps/outlook/data-files/shared-mail-folders-in-cached-exchange-mode 

Apart from the reg keys mentioned in the article above, you can consider adding the shared mailbox as additional account in Outlook, instead of having it as additional mailbox/automapped. The steps for that are here: https://michev.info/blog/post/3567/how-to-add-a-shared-mailbox-as-additional-account-in-outlook-2022-version