This appears to be an ongoing issue for the past several years and I am still experiencing what so many others have found. There is a problem printing a LARGE (100+) PDF with mixed legal and letter sized pages using Adobe Reader on a Mac with a Brother laser printer.

I think that Adobe support has not been able to recreate the issue because their test file has only two pages. If they create a 100+ page test file with mixed letter and legal and they should be able to replicate the issue.

Here is what I have:

I have a MacBook Pro (15-inch, 2016) running Catalina, Brother HL-L6200DWT printer and using Adobe Acrobat Reader DC 2020.009.20063. I have all of the settings set correctly per the Adobe: https://helpx.adobe.com/acrobat/kb/print-mixed-page-sizes-acrobat.html
I contacted Brother support as well and followed the details found at : https://help.brother-usa.com/app/answers/detail/a_id/102267
This included downloading and using the latest drivers from Brother. As instructed I tried both their CUPS driver and the BR-Script driver.
I have printed using both a direct USB cable and over WiFi.

When I print a large (100+) PDF file with mixed legal and letter it initially works for the first 10 or 20 pages but eventually the pages get cut off with legal pages being printed half way down the page and/or cut off. It is as though the data being sent to the printer eventually becomes "corrupted" so that it is unable to print correctly.

Brother support provided a workaround that does work: Rotate the view of all of the pages by 180°, so it looks upside down on the screen.
Then print the document as usual, and all the pages should print completely.

Attached is a test.pdf file of multiple mixed size pages that I used to replicate the issue.
Also attached is a result.pdf which is a scanned result from the printing showing the issue with pages being cut-off / misprinted. Note that the A4 pages are scanned as legal size, but you can still size how they actually printed on the actual paper.