How do I AirDrop from a MacBook to its tethered iPhone?
You will need to turn off Personal Hotspot before you can AirDrop.

AirDrop transfers data over Wi-Fi between the two peer devices (in this case, your MacBook and your iPhone). Since your iPhone is providing a Personal Hotspot via Wi-Fi, that connection is not available for AirDrop.

If turning Personal Hotspot off and on repeatedly is too cumbersome, you can provide tethering through USB instead. Then your Wi-Fi connection will always be available for AirDrop.