Exported .pst loses a year of mail?

I want to migrate my website to a new hosting provider (need a new site anyway as K2 won't work with Joomla 5).

To keep my mail, it appears my only option is to use an IMAP client, e.g. Outlook.

I want to export my mail as .pst, set my account to the new mail server, and import the .pst.

But every time I attempt this and import the .pst, I lose exactly 1 year of mail. Nothing after early March 2024 appears.

I have no idea why.