What does my immigration stamp mean if it doesn't indicate a 30-day visa exemption?

The stamp you received upon re-entering Thailand reflects a valid extension from a previous education visa, which expires on July 22, 2022. Since you had a single re-entry permit, the immigration officer stamped you based on that extension rather than granting a typical 30-day visa exemption. It's important to note that you should have canceled your extension with the immigration office before leaving Thailand to avoid complications, but currently, you are allowed to stay until the expiration date.