Macbook trackpad erratic when plugged in

It's odd that I don't see this issue on the community because I have experienced this with a 2018 Macbook (gone now) and with a 2015 Macbook Pro (both have haptic trackpads). Basically, when the pro is plugged in and charging, my trackpad mostly can't control the movement of the of the pointer and gets worse over time. It took me three (!) trips to Apple support to figure this out on my own - even Apple didn't find this problem. Each time I took the device in, without a charger of course, the behavior was normal. So the first time, they conducted the usual troubleshooting and sent me home. The second time, they kept it and re-installed the OS. The third time, they sent it off for a rebuild (replaced the keyboard/trackpad unit). And of course, it worked fine each time until I got it home. I even suspected my network, so disabled network and bluetooth as part of my own troubleshooting. But now I can confirm that I plug in and it is noticeably hard to land the pointer - unplug it, the backlight dims a bit and the trackpad works fine.

So I'm going to take it back in, but first I wanted to post this in the community to see if anyone has any thoughts on this.