Not all cables are made equal, while I do think Monster Cable is overpriced and overrated, you don't want to use cheap cables for anything more than a reasonable run - many consumer AV applications fall into that category.
I usually use Amazon Basics cables for most of my needs, I believe they come from the major Chinese/Taiwanese cable suppliers(Space Shuttle, Copartner). I've used their 6' TOSLINKs for my parent's home theater setup and it seems to work fine.
I think the splitter might have caused trouble with longer cables - ideally each device gets a separate feed. For a soundbar, there might be a set of RCAs or another TOSLINK out - better ones have Bluetooth built in for streaming.