My guess would be that it possibly has something to do with how all the apps on SkyOS are stored on Sky's servers, not on the Glass TV or Stream puck itself.
iPlayer is one of the apps which has to check on the display's capabilities upon launching in order to determine whether it can provide UHD with HLG or not. It's possible that some sort of network issue (either on Sky's network or the customer's network) possibly prevents this check being fully completed. This may then force the output of iPlayer to a lower bit-rate, and potentially in the wrong colour space. This could be the cause of the washed out look you were getting @Anonymous
HLG also takes its cue from the peak brightness of the display being used. Glass gen2 is a reasonably bright TV, even Vincent from HDTV Test confirms this, so it should be able to display UHD HLG HDR iPlayer output very well, as @stereohaven was able to see yesterday.
TBH, I just think it could be another random glitch in SkyOS. It's clear that the OS can suffer from sluggishness, again due to being server-based, so added delays in hardware spec being reported to an app in order to output correctly are to be expected.
This is why external HDMI connected devices like an Apple TV box don't suffer the same problem - they are not relying on sending a communication signal from a server to a device and back again in order to determine an output specification. A Firestick, Apple TV, Roku, Android TV (or whatever type of box or stick you have attached to the HDMI port on Glass) is making the communication directly through the HDMI port. This is much faster and therefore more reliable than an app on a server.
