Bad news...
I currently have one ObserverIP with 4.5.1 and another with 4.5.2. I didn't see this at first but now I'm seeing the one running 4.5.2 reporting Absolute pressure instead of Relative but just to WU, as it seems fine at AmbientWeather.net. So perhaps this is a firmware bug in 4.5.2. I've reported it to Ambient Weather. For now I recommend running 4.5.1.
4.5.2 is supposed to fix some UV scaling errors and rain reset problem with daily rain. These might just be internal issues or intermittent as I've not seen it be a problem. So for now I'm sticking to 4.5.1 for my Meteobridge connected ObserverIP.
In my testing with my WU station ID with 4.5.2 firmware I see the pressure change just as the page loads. It changes from proper Relative pressure down to Absolute pressure.
KFLWINTE860 - 4.5.2 - problems with Relative pressure
KFLWINTE862 - 4.5.1 - No problems
Both ObserverIP's are getting the same weather data from the same indoor pressure sensor and they are calibrated the same. So the problem is in the ObserverIP firmware.