So I was doing Grimrail Depot +4 and the party skipped a pack in the beginning. This led us to 98% and the last boss didn’t spawn at all, so we couldn’t complete the mythic and my key was wasted.
Has anyone else experienced this? Because we couldn’t find a way to get back before jumping on the train.
A bug report would be the right course of action here. While skipping too many mob packs leading to insufficient completion ratios is obviously meant to happen, it should generally speaking still be possible to complete the dungeon. 
You can return to the beginning by using a hanging chain at the start of the train. We encountered the same problem - Tovra didn’t spawn at 98 %, we returned to the beginning, killed the pack we missed, and the last boss was there when we came back to her.