Mysterious quality drop after exporting

Hello, I'm a bit of an amateur when it comes to video editing and I could use some help. I recently created a montage from video game recorded from my PC. I use Nvidia Shadowplay to record a monitor capture at 1280x720, 60 fps, at 50mbps bit rate. My sequence settings on Premiere match this and everything looks great in the preview. I render and export using H.264, preset settings are for Youtube 720pHD, profile is set to high with level 4.2, and I have the encoding set for 2 passes with a target bit rate of 60. Everything I've read from guides tells me that my videos should be coming out of the export looking pretty good, but I'm noticing a very clear drop in quality that I can't seem to find an explanation to.

I use an effects pack for some touch ups and transitions during the video, and I considered if they would be the cause of the problem because the effects themselves are in a 4k resolution. Scaling them down to match the 1280x720 video didn't help the quality, and neither did removing them entirely. Without the effects I then tried exporting at the highest possible settings, 300 bitrate, etc, and I'm still not seeing any increase in quality.

I don't understand if I'm doing something wrong or if there's simply some setting I overlooked, but any help with this would be appreciated.
Here is the video in question that I'm experiencing issues with. At 2:21 you can clearly see what looks like the video suffering from a low bitrate: ULTRANUMB | A Rocket League Montage - YouTube