Thank you for reaching out on Microsoft Q&A forum.
Yes, Azure Front Door Standard Tier does not support Managed rule-set it onle supports Custom Web Application Firewall (WAF) rules. The work around in this case will be to add managed WAF rules manually after migration or to use Premium tier of Azure Front Door as it supports managed rule set. This is currently documented here.
What is the difference between a front door(standard/premium) and a classic front door.
You can go through this article to learn more about the features supported by each tier.
Hope this helps! Please let me know if you have any additional questions. Thank you!