Insanely slow 2Degree data speeds when roaming onto the Vodafone cell site on Great Barrier Island

We visit Great Barrier Island a few times each year, and being 2D mobile customers, we roam onto Vodafone's cell site as 2D don't have coverage in Claris, or maybe anywhere on Great Barrier. In the past we've received fairly reasonable data speeds, but I've noticed during the last few visits in the last 3 months, our data speeds when roaming there have dropped to around 0.9Mbits/sec (measured using the Ookla app). No 3G or 4G indication or even H+. This has been confirmed with 2x phones and a wireless 3G router. Two other visitors at the house with VF SIM cards received 100Mb+.

It seems that 2D customers are being treated as 3rd class citizens, and I wonder if this is normal or widespread when roaming. The current speeds are so bad, it looks like we're going to look at jumping to the red company I despise. Does anyone know if this is normal or are VF pulling a swifty?

The 2D maps show our area to have "3G Boosted" performance. Phones being used are Galaxy S7, S4, and S8.