using RDP with BTT

I access a work PC on my personal iMac using Microsoft Remote Desktop. I use BTT to set mouse buttons 3 & 4 for web browser navigation. On the PC, navigation is controlled with Alt+arrow keys. After some experimentation, it seems that the Alt/Option keypress is being ignored and only the arrow key is coming through.

The Alt key works fine on the keyboard; it's the translated mapping through BTT that is failing. Is this something that anyone else experiences? Can anyone think of a workaround that gets Alt+arrows working in RDP?