So this isn't a Fusion issue, well not directly. I've seen it before with other CPU/GPU intensive applications, particularly on older machines. Encoding video is another common use case.
There's basically two causes:
First, using a third-party, counterfeit, undersized, or failing power supply, which causes less power to be delivered than the machine consumes. Make sure you're using a real Apple 85w supply with that machine, and check the mag-safe connector. If you see black there, from arcing, power off the machine and use a pencil eraser to clean the connections on both the supply and the computer. Also check to make sure there's nothing stuck in the magnetic latch on the computer - my mom had a staple stuck in there that was causing intermittent connections. Likewise, if the cord on the supply is bent, bulged, etc (common if you use the wings), try getting a new power supply.
Second, a machine that overheats. The root cause here is Apple trying to stuff too much hardware into too thin a case, but made worse by dirty/failed/clogged fans (open the case and use canned air to clean), or using aftermarket software to control fan speeds. Running with the lid closed, or putting the machine on a heat-absorbing surface (e.g. a couch) *absolutely* causes overheating, which will throttle charging. Best option is to put the machine up on a stand, blow a fan on it, and use with the lid open.
