This is neither a bug nor a feature, just a happenstance of the new tier populating.
As Plarium was explaining it to me, because G5 is not yet stable/fully populated, the matchmaking is ranking you against a wider array of people. This means for a bit you may see a range of +4 to +16. and as it settles you will naturally see more +6 to +14, and eventually even something like +7 to +12 as being the natural range of opponents.
Basically, it will settle out over a couple resets.
Side note to Player J, if you fight the people with a Higher Arena Rating than your current rating, at the BOTTOM of a page, you can gain 16 points a fight. If you lose to the people with the lowest Arena Rating, at the TOP of your page, you may lose 16 points. Basically, if you are losing to Lower Rated Opponents, the "easy" ones, you're going to lose more points than if you lose against the Higher Rated Opponents, the "harder" ones.
I solved this problem by focusing on hitting teams with a higher Arena ranking than me, and not losing against teams that aren't even technically in Gold 5, ranked below me, such as 3300-3500. Doing that, I have had no issue staying in Gold 5 myself.
TLDR: Lose less against teams with a lower rating than you, and you won't have to waste 400 gems.