Very likely it's high bitdepth. L-Smash outputs the hacked format by default, then. You can let it dither to 8 bit:
Code:
lwlibavvideosource("source.ts", format="YUV420P8")
Or put into hacked stacked format:
Code:
lwlibavvideosource("source.ts", format="YUV420P16", stacked=true)
Into AviSynth+ native format:
Code:
lwlibavvideosource("source.ts")
ConvertFromDoubleWidth(bits=10)
See README for more info.
Compatibility of non-8 bit formats can be limited in older software, e.g. VirtualDub.
ffms2 does all this automatically, btw. Easier to use.