Power BI is throttling data loading

I doubt the problem is on the PowerBI side of the process.

Check the API documentation.  A lot of times there is a limit to the amount of data an API will return in 1 call.  You may need to change the page size parameter or even deal with getting a continuation token.  

Well, I would have  thought it would have  followed the pattern of 1000 rather than 400.   Check to see if it is Paging.

With the retirement of Outlook Calendar Boards, this topic is of interest to me.

What are the results of testing your query(s) with Graph Explorer?   What does the process documentation say from when you put the logic together for this originally?

I found a potential issue in the API for this case, which are limiting queries to "400", but I can't edit the API code. Do you know how to edit the code or change it to unlimited?