Escape key in putty not working

I’m connecting to my TS 16 via SSH with Putty. For some reason when I hit the ‘Escape’ key it’s not being sent to my devices, a critical thing because it’s the escape key that allows me to interrupt the boot sequence.

Other characters seem to work. I wonder if it’s the terminal emulation? It’s currently set to VT100+. I also noticed that the backspace key does not work when connected directly to the terminal server, is this normal?

What ‘port profile’ is configured for the serial ports in question?