New Outlook not auto-syncing on Laptop

Question

I'm an Azure Cloud Engineer for my company.

Recently a staff member has started using New Outlook and after a week it stopped automatically syncing with Exchange online.

• No one else has this issue, and the user is able to still receive emails real-time on their mobile device.
• I have found one other post online about this issue, and followed their fix, however it only resolves the issue for one day before it reoccurs.
• This fix was to go into Settings > Accounts > Email accounts and then to Remove their account. After opening New Outlook again, their email would work and sync for about 1 day.
• They are only using 3% of their assigned inbox size, so it's not that they have too much email.
• Their version of New Outlook is up to date and matches everyone else in the organisation.
• I cleared away their older %localappdata%\Microsoft\Outlook folder, but that does not appear to be attached to New Outlook at all.

Any suggestions or known fixes to this issue?