Emails disappear after transfer from iPhone 7 to iPhone XS

The problem is related to transferring all my emails (over 25,000 of them) from iPhone 7 to iPhone XS.

After the transfer of all settings and data (either via restoring back up at my Mac or stored in iCloud - both variants led me to the same results) - the number of emails indicated on the new iPhone is correct - over 25,000. And all mailboxes on the new iPhone XS contain required information. However, after a while (I don’t know why - either new iPhone starts updating mailboxes or due to some other reason) - all mails from mailboxes are deleted and the number indicated on the new iPhone is reduced to zero.

Could you, please, recommend me what I should do to avoid losing my emails?