iCloud mail - Emails not available

This is my partner’s ICloud email account.

She created a folder Work, where she moved all work related emails, and this was last used around 2 years ago.

She recently needed to access these emails but the Work folder is empty.

Why are these emails not visible in the folder?

How can the emails be recovered?

Not sure if relevant, but when I look at the Sent folder, the earliest email is Jan 2024. I would expect to see Sent emails dating back several years