Hi
@kemp990. Great to see some riders from over in Seoul (I spent a month there a few years ago, great city! but Jeju Island was my favorite).
I rarely use the walk assist button but when I do, it has always worked fine for me. However, I don't really lean on the bike so maybe that is why. Since it is night time here (11pm) and I'm leaving for the weekend I just tried my bike indoors really quickly. I found something that may help. When I used the lowest gears (i.e., the 40 or 42 tooth gears) and tried the walk assist button, and then restricted the wheel from rotating it seemed to go into a 'low-power' mode like you described. But if I went up a few gears to say a 20 tooth gear everything was great and consistent. I also just found out you can even change gears without pedaling if you use the walk assist button
Do you recall if you were in the 40 or 42 tooth sprocket when going uphill? If so, next time can you try changing gears to a mid-gear and see if things are normal?