Per the suggestion from mklement0 I have shortened the output here to just the relevant code and error. Additionally, as he pointed out, I have changed how I am constructing the arguments (now passing as an array).
Additionally, the comment from kesh, I removed the "-filter_complex" and changed to just several "-map" 's.
$TEMPDIR = "E:\MediaConversions\BitLadder"
$FOLDER = 'White.Boy.Rick.2018.1080p.WEB-DL.H264.AC3-EVO'
$OUTPUTDIR = $TEMPDIR + '\' + $FOLDER
# Mux the 8 Bitrate Ladder steps into a single file, with a common audio stream
$STEPS = @(Get-ChildItem -Path $OUTPUTDIR -Name)
$INPUT_FILES = @()
$MAP = @()
$COUNT = 0
ForEach ($STEP in $STEPS){
$INPUT_FILES += ('-i', "$STEP")
$MAP += ('-map ' + "$COUNT" + ':v', '-map ' + "$COUNT" + ':a')
$COUNT++
}
$OPTIONS = ('-' + "$INPUT_FILES", "$MAP")
#$OPTIONS
D:\Applications\ffmpeg\bin\ffmpeg.exe "$OPTIONS" -f matroska "E:\MediaConversions\White.Boy.Rick.2018_MultiStreamTest_AAC.H264.mkv"
But Sadly, I still get the same frustrating error:
Unrecognized option '-i White.Boy.Rick.2018.1080p.WEB-DL.H264.AC3-EVO_STEP1.mp4 -i White.Boy.Rick.2018.1080p.WEB-DL.H264.AC3-EVO_STEP2.mp4 -i White.Boy.Rick.2018.1080p.WEB-DL.H264.AC3-EVO_STEP3.mp4 -i White.Boy.Rick.2018.1080p.WEB-DL.H264.AC3-EVO_STEP4.mp4 -i White.Boy.Rick.2018.1080p.WEB-DL.H264.AC3-EVO_STEP5.mp4 -i White.Boy.Rick.2018.1080p.WEB-DL.H264.AC3-EVO_STEP6.mp4 -i White.Boy.Rick.2018.1080p.WEB-DL.H264.AC3-EVO_STEP7.mp4 -i White.Boy.Rick.2018.1080p.WEB-DL.H264.AC3-EVO_STEP8.mp4 -map 0:v -map 0:a -map 1:v -map 1:a -map 2:v -map 2:a -map 3:v -map 3:a -map 4:v -map 4:a -map 5:v -map 5:a -map 6:v
-map 6:a -map 7:v -map 7:a'.
Error splitting the argument list: Option not found