Media Encoder (24.3) - Black and White screen GLITCH when exporting H.264

This is my first time posting one of these types of questions. I know how detailed people need to be in order to get informed responses so please bear with me in the comments as I get back to you the computer specs that are requested. For now, here is the issue: Only recently have my h.264 exports had this weird black and white glitch that FLASHES at random. It doesn't seem to show up in Premiere Pro (though I have seen it once) so it's not an issue on the timeline. This is the before...

https://drive.google.com/file/d/1motgUhyqbtjynKSE7Btt5R0FYTu-Ifyo/view?usp=sharing

These are the after...

https://drive.google.com/file/d/1ArbcHcNh6bQ8hErbZMhE2ATQFzoLvR3H/view?usp=sharing

https://drive.google.com/file/d/1yPH5L2gUpsEYT-or5DHcWPCJdsjas83x/view?usp=sharing

Here are the preset specs I HAVE been using (yielding the glitch)...

https://drive.google.com/file/d/1MIEGWwW-h8aQHskNfZAMgajbQ-vxdkYX/view?usp=sharing

And these are the preset specs I TESTED that seemed to be glitch free...

https://drive.google.com/file/d/1YgApE9UJygwT86B_TJdEDNn-8vrwmWEg/view?usp=sharing

Something seems to change between the "GLITCHY 1280x720 preset" and the "NON-GLITCHY 4K preset" but my untrained eye can't seem to figure out the root issue and I'd love to be able to export the smaller-sized videos again without the glitches so let me know what I can look into to figure this out.

System Specs...

• Processor: 3.8 GHz 8-Core Intel Core i7

• Graphics: AMD Radeon Pro 5500 XT 8 GB

• Memory: 128 GB 2133 MHz DDR4

• Startup Disk: Macintosh HD

• MacOS: Sonoma 14.4.1

Software Specs...

• Premiere Pro 24.3

• Media Encoder 24.3

Footage Specs...

• Kind: QuickTime Movie

• Dimensions: 3840x2160

• Codecs: Timecode, Linear PCM, H.264

• Frame Rate: 29.97

Order of Issues...

• Editing in Premiere Pro (no issues)

• Original files all 3840x2160 .MOV with 29.97 Frame Rate

• Edited in a 3840x2160 timeline with 29.97 Frame Rate

• Sent to Media Encoder with 1280x720 aspect ratio, H.264 Format, and 29.97 Frame Rate

• Final MP4 has BLACK AND WHITE GLITCHES

Again, there are going to be questions about specs that will remind me of other differences so please be patient with me as I answer any inquires in the comments. Thank you so much.

SMALL UPDATE: I'm noticing the issue might be specifically linked with the aspect ratio as I exported using the 4K settings but changing ONLY the aspect ratio and the glitches returned with a vengeance. Still experimenting.

SECOND UPDATE: While every version of 1280x720 h.264 export I tried through Media Encoder yielded the glitch, I switched the Renderer to Mercury Playback Engine Software Only and the glitching seems to have disappeared. Is there an obvious root issue here? And can I eventually switch back to GPU Acceleration or will I have to endure the x2 exporting time from now on to avoid glitches? I wanna try and figure out EXACTLY what's happening.

THIRD UPDATE: I took my iMac in to Apple to have it evaluated and they said the Graphics Card isn't showing any signs of deterioration. Not sure what has changed all of a sudden. I uninstalled Adobe and got rid of all the presets and folders I could find but the issue persits. The only thing I haven't done yet is Reset the entire computer to factory settings.