Certain clips becomes pixelated and flickering in the timeline and in the export

Certain clips become pixelated and flickering in the timeline and the export, while certain clips look okay. For example, Clip A in the table below has this problem.

My sequence settings:

• General
  Editing mode: Custom
  Timebase: 23.976 fps
• Video Settings
  Frame size: 1920h 1080v (1.0000)
  Frame rate: 23.976 frames/second
  Pixel Aspect Ratio: Square Pixels (1.0)
  Fields: No Fields (Progressive Scan)
• Audio Settings
  Sample rate: 48000 samples/second
• Color Space
  Name: BT.709 RGB Full
• Default Sequence
  Total video tracks: 4
  Mix track type: Stereo
  Audio Tracks:
  Audio 1: Standard
  Audio 2: Standard
  Audio 3: Standard
  Audio 4: Standard

My Premiere Pro 2021:

• Version 15.1.0 (Build 48)

My System:

• Windows 10 Pro
• Processor: Intel(R) Core(TM) i3-2120 CPU @ 3.30GHz 3.30 GHz
• Installed Memory (RAM): 8.00 GB
• System Type: 64-bit Operating System, x64-based processor

My Export Settings (may not be relevant as the problem exists even in the project timeline itself):

Preferences Settings:

Media > Default Media Scaling > Set to Frame Size.

I have tried to change the Default Media Scaling to "None" on a fresh new project but the clip still looks pixelated.

It seems like the only way I can get this clip in Premiere Pro without looking pixelated is by dropping this clip as the first clip into the Sequence and choose "Change sequence settings" to match the clip's settings. Doing this, however, will cause the other clips to be cropped out since they have a 16:9 aspect ratio instead of 1:1 (square) like this clip.

I can't seem to find the reason why some clips are having this problem while others don't. It seems to be a recurring issue for many projects.

Any help is much appreciated. Thanks!