Preflight complains of wrong URL when behind reverse proxy

[Computroniks]

I have been trying to setup Snipe-IT behind an SSL terminating reverse proxy. I have set the APP_URL to https://snipeit.example.com as well as the APP_TRUSTED_PROXIES to the IPv4 and IPv6 addresses of my proxy. On the proxy I have also set the X-Forwarded-Proto header to be set for HTTPS traffic and verified it in a packet capture. When I navigate to https://snipeit.example.com I am rediected to the setup screen (https://snipeit.example.com/setup). Here everything is green apart from the URL where is says:

Uh oh! Snipe-IT thinks your URL is https://snipeit.example.com, but your real URL is http://snipeit.example.com/setup Please update your APP_URL settings in your .env file

I assume that I have probably missed some sort of config for this but I can't figure out what it is.

System details
New install of Snipe IT following git clone instructions in docs
Snipe IT version: Pulled from master at commit 89fee23

Thanks for any help