Sent emails disappeared

Today I needed to rescue an email I sent last year. But I noticed that the Sent folder on my Mail.app only shows messages from March 2021 and later. Looks like all messages before March 2021 have vanished. I've checked the Sent folder on iCloud online and it's showing the same messages.

What happened to my old sent messages? Are they gone for good? Is there a setting that I'm not aware of which is causing sent emails to be deleted?

Any help will much appreciated. I'm quite in shock about this since I use my email for 10+ years without issues.