I have encountered such a problem. At that time, I found the problem because I changed the version of outlook. I used the English version of outlook on my old computer. After copying the old database on the new computer, for convenience, according to the same version of Chinese outlook, I found that the mail disappeared. But this problem was solved after using the same version of outlook as in the past.

My advice to you is to install the same version of outlook as before, and then use the "check for updates" function that comes with outlook to automatically upgrade online. This can avoid problems with the database.