I'm glad that you were able to resolve your issue and thank you for posting your solution so that others experiencing the same thing can easily reference this! Since the Microsoft Q&A community has a policy that "The question author cannot accept their own answer. They can only accept answers by others ", I'll repost your solution in case you'd like to accept the answer.
Ask: Need help recovering access to Microsoft Learn Sandbox
Solution: I followed the steps outlined in the documentation (Leave an organization where you have a guest account) and was able to resolve the issue. After a 30-minute waiting period, I rejoined the Microsoft Learn Sandbox directory and accessed it without any issues.
Please don’t forget to Accept Answer and Yes for "was this answer helpful" wherever the information provided helps you, this can be beneficial to other community members.