Hello Xigua!
Thank you for raising the question.
There are the same rate limits applying to MCR as they apply for ACR. However, please note that MCR a free service which doesn't provide any SLA so a theory may be the fact that you've encountered some throttling.
I hope this is helpful.
Please "Accept as Answer" and Upvote if it helped, so that it can help others in the community looking for help on similar topics.
To expand on your issue and share the feedback internally, could you please help with the following information?
1 - What error did you get?
2 - Please provide the exact timestamp(s) in UTC for when the issue happened.
3 - What images did you try to pull?
4 - Did you conclude any pattern?
5 - Feel free to share any other information that you think it may be related or useful (make sure not to send any confidential information)
Thank you!