The lack of responses to this question does not surprise me.
I am having a difficult time coming up with any answers for you as well.
Tank water heaters can indicate problems of the very recent past. Tankless are a flow through design with no storage, so problem water that occurs there should also occur on the cold water.
I can only assume your tankless water heater is having a problem of some sort, and the flavor is coming from it and not your incoming water. Unless the rapid rise in temperature is causing the problem. Turn off the tankless and run hot water to see if it is the same bad taste regardless of if heating is on or not. Then turn on the heat and see if the problem returns.