I want to know why I don’t see projects after transferring them to another organization (including when they seem missing from both orgs).

After a transfer is accepted, the project is in the destination org and is removed from the source. Teams do not transfer with the project, so it may not appear in the Projects list or project filter until you add a team.

First, confirm that the destination Owner accepted the transfer email.

Then, in the destination org, go to Settings → Projects → [Project] → Project Teams and add a team to the project. If Open Membership is off, you’ll also need to join that team.

Managers and Owners can still open the project under Settings → Projects even before a team is linked.

See Transfer a Project and How can I transfer projects between Sentry organisations?