Hopefully this saves some of you some time troubleshooting this.
When loading in music through the "Local Files" feature, Spotify does NOT automatically load in files with the .MP3 extension, but does load in files with the .mp3 extension.
After several rounds of trying to figure out why an album was not showing up properly despite others in the same folder working just fine, I noticed the difference in file extensions. Simply changing the file extension to the correct variant enabled the files to be detected and loaded.
Interestingly, the day before I discovered this I attempted the same procedure the .MP3 files WERE loaded in, but then began to appear as unavailable on my phone after I had downloaded the playlist. My troubleshooting began here. I'm not sure how or why I was able to load them in on the previous day, maybe something on the backend.