Content caching doesn't work in Catalina?

I have a 2014 mac mini setup as an always on computer with content caching enabled. This worked very well when it ran Mojave in combination with the following computers:

iMac 2017 (Catalina beta)

Macbook Pro 2015 (Mojave)

But now that I have updated all the computers to Catalina, content caching no longer works. I even downgraded my Macbook Pro to Mojave again, for 32bit reasons, which suggest the problem lies with the Mac Mini somehow.

I've reset the cache on the machine, but when I download an app from appstore using any of the other computers it isn't being cached on the Mini.

If I download the app on the Mini first the app IS cached, but if I then try and download the same app on the other computers it isn't fed to them from cache :/

I've tried disabling and even quitting my VPN software on all machines, and various power cycling too, but it doesn't help.