did you use MKVToolnix by it's self or in streamfab - settings - vip services - under video format - MKV (MKVToolnix)
october262
I downloaded first, as MP4, then created the MKV with the +7s delay in MKVToolnix. Oddly this only affects the audio. I am now reviewing other content downloaded from there, I suspect they must be affected by this, too. I checked the videos after adding the positive delay, and they are in sync.