*** Solved ***
I did some troubleshooting using XCTU and converted my AP parameter on my board from MicroPython to API. Then with my Xbee Micro development board, I sent a Tx Request: 64-bit address to my board. It received the message in 16-bit mode, not 64-bit as expected.
I looked at the MY parameter, and it was set to 0x0000 somehow. I changed it to 0xFFFF on all of my boards, and then was able to retrieve the 64-bit address of the sender.