Odd binding problem

Metabase 0.50.16
I'm launching a dev environment from a batch file in Windows.
I have

SET MB_JETTY_PORT=3000
SET MB_DB_HOST=192.168.3.157
SET MB_JETTY_SSL=false

plus all the other options you'd expect.
For some reason, it fails to launch as there's a conflict on port 443. I also have IIS running on 443. If I stop that, Metabase launches correctly AND it's using port 3000 as expected! I can then restart IIS and everything works correctly.

Any ideas what could be going on?