403 Error When Adding Videos via YouTube Analytics API to Groups with 200+ Items, Despite 500-Item Limit

I'm encountering an issue with the YouTube Analytics API's groupItems.insert method. According to the official documentation, a group can contain up to 500 items. However, when attempting to add videos to a group that already contains 200 or more items, I receive the following error:

403: Group contains maximum number of items.

Interestingly, adding videos manually through the YouTube interface allows up to 500 items per group without any issues. Notably, the API revision history indicates that the maximum group size was previously increased from 200 to 500 items.

I have the following questions:

1. Is there an undocumented limitation when adding videos to groups via the API that differs from the stated 500-item limit?
2. Why does the 403 error occur when the group contains fewer than 500 items?
3. Are there any known workarounds or best practices to address this issue?

Any insights or assistance would be greatly appreciated.

Thank you in advance.