I agree with Everynone, somewhere in the tick you set the rotation.
Show the blueprint so we can help you.
As you can see in this video, my pawn constantly faces one direction. Any help or ideas on why this is happening?
Looks like you’re overriding it somewhere in tick. Hard to tell without seeing your setup.
Does this help?
Also, I’m using the “Custom Gravity” Plugin, if that helps as well.
The video does not help, it’s just some disorganised uncommented blueprint spaghetti. 
Generally speaking, characters walk pretty straight in UE4 so I’d blame the plugin at this point.
Consider adding this information in bold in the topic - perhaps you’ll be lucky enough to attract attention of someone knowledgeable / familiar with the inner workings of the plugin.
Can’t help you, sorry.
Ah, Shame. My workflow is commonly very unorganised anyways. Thanks for trying, though. I’ll keep trying to fix it, and hopefully find something.
Okay, making the rotation smooth apparently fixed it. Cool?
even thoug you fixed it. I’d like to know what the “add camera yaw input” and “add camera pitch input” finctions do in your “custom pawn” do, They might be the source of your initial problem. I mostly use “add controller yaw input” and “add controller pitch input”.
Not quite sure. I think their default for the pawn included with this plugin, but i assume they are meant for seperate camera control