I have same problem.
For ppl who just need map completion for another characters without doing story on each character – you can reach last POI in “The bitter cold” just use Thermal tube on the north of “Dragons teeth hot springs”. This tube does shoot you under waterfall close to the Mastery point in the cold zone, just fast jump down for POI and die on the frost
But you will have Map completion.
For ppl who need this for daily use – take your main character again to the story. After what you reach story to the point where grawl got to you free Grawl firestone, you can buy more pieces at Warming Sun heart-vendor. I already bought 50 pcs for future progress right this way. Nothing against Anet but I know how long time can take this issue solve. But very important is buy Grawl firestones during this story line, after what you already cooked elixir and finished story line Warming Sun no longer will have this item in the inventory!
For ppl which needs achievements – unfortunately, the only way how working in the Cold zone is do it again through story with new elixir. Notice that elixir working whole day until daily server restart, so start as soon as possible during day or buy more Firestones during story line.
Hope it helps and bug will be fixed asap.