The charger doesn’t offer enough juice to run your system hard. Basically, your systems usage wattage is exceeding what the charger can supply alone, you need the battery.
Here’s another example of this issue the top end 2016 15” & 2019 16” systems with battery couldn’t render 90 mins of Red video even with a charged battery and charger plugged in without kicking the bucket! The only way was to put the system into a freezer to keep the system chilled! That way the process job would take less time.
[https://www.youtube.com/watch?v=Dx8J125s4cg&t=112s|MacBook Pro 15 (2018) - Beware the Core i9]