GPS distance running way off (Latest software installed)

What does the GPS trace look like on the map? Is it jumping around or does it look to follow the route you took?

I’m always amazed by the accuracy and repeatability of the GPS on my 955. The only place it really struggles is downtown when I’m surrounded by hall buildings. Even then it does a pretty good job though. 

It looks super clean

I don't know how or what, but something has changed drastically :-(

Odd. Only other suggestion from me would be a restart. Turn the watch off and on and see if that helps. 

Did a track run today with 800 mtrs intervals. A manually recorded the laps and not using the track run but the regular runapp. They were spot on exept the first interval. 
connect.garmin.com/.../11641840303

3d checked and ok, and I do use hrm-pro, but I don't understand what you wrote :-/

@SMR - Found the settings (it had to be connected first) it was set to "always" - changed it to "indoor". Exited to see what happens tomorrow for my next run :-)

I do parkrun regularly.

My regular parkrun course has lots of tree cover and has many 90° bends in quick succession.

That always measured about 20% short on my vivoactive3, depending on conditions. With the 955 it usually measures about 10% long.

I suspect that the course was not correctly measured with a wheel, but with a GPS device instead.

I do a different parkrun on a beach: out and back along the shoreline. No obstructions to the sky, no turns except the one at the half way point. That always registers exactly 5km ± 50m, on 955 and va3.

If there's any tree cover, make sure you have GPS set to All + Multiband, and set it to record once per second rather than the default of "Smart". Turn off 3D distance/pace too.