So this is the line that is triggering the error. It appears to be having a problem with that instance of mongoose.
There is a whole thread about transactions using Fawn (which has not been updated in years) and some code for changing this to just skip Fawn entirely (preferring to use mongoose transactions directly): [Mongoose] Transactions vs Fawn (Two Phase Commits)
I think that the basic issue here is just that Fawn does not know how to recognize the current instance of mongoose since it has not been updated in so long.