@nas I don’t have conclusive results but it looks like it is related to whether the screen is pointing at the body. It does not always have an effect though. Maybe this only happens when additionally not many GPS satellites are available or the connection to one is lost. I need to observe this for a longer period. Maybe others could also check whether the GPS accuracy remains good when the phone is in the pocket and the screen is pointing away from the body.
@msknight this might also be interesting for you