Hi Ronin,
This one I cannot replicate. I've imported 9000 contacts and upgraded but they are all still there.
If you have a 3CX Backup (not just a contact export) you can confirm that the contacts are there, you can restore the 3CX Backup to get them back
PS: Do keep in mind that Alpha and Beta releases are not meant for production, only for evaluation. You should always have 3CX backups going forward - especially before an upgrade of this type.