Symbols With Same Tween Moving at Different Paces

I'm building a puppet in Animate 2019, and today I was doing some test animation with it when I had to stop because of this problem. I was trying to tween the character's arm to raise above his head. I made two keyframes, made sure none of the pivot points were changed between the two (because I know sometimes that can also cause trouble), and set all 3 symbols to Classic Tween. However, the hand, upper arm and lower arm are all moving at different paces even though they are all set to Classic Tween. Does this mean something else is wrong with the puppet that's causing it to do this? I sent the file to my professor and he got the same issue so we know my Mac isn't what's causing the problem, otherwise it would've worked when he tried it from his own computer. Could it mean the master puppet got corrupted somehow?