When I play the RuneScape MIDI files, some of the instruments, like the French Horn, only plays for a split second. When playing the files using the Microsoft's GS/GM Synth, it works, same with the Yamaha TG100. That, and when there's an unknown rhythm, it doesn't have anything for the rhythm channel (Reggae.mid) until I manually change it, or when the waves play in the Last Wave (OutRun) MIDI file, it takes a while for the wave noise to start (and when the note gets released and starts up again, it doesn't want to play nice).
Well, the good news is that, based on at least two of the three examples given, your SC-55 seems to be behaving exactly as designed.
As derSammler mentioned, RuneScape is ~10-years newer than the SC-55. Its music definitely doesn't seem to have been composed or optimized with a 24-voice polyphony limitation in mind, resulting in the premature note cutoffs you've experienced when voice-prioritization/stealing occurs. I would suggest that a single SC-55 is a poor candidate for the playback of music from this game.
Regarding "reggae.mid," and assuming you're referring to the 1991-dated, Voyetra Technologies sequence, Program Change 127 is sent on Channel 10 to select the drum set. This is not a valid selection on the SC-55, and results in - yes - silence, where the SC-55mkII-based (and later) architectures will simply use the last valid drum set selection instead. It's apparent that Program Change 128, selecting the CM-64/32L drum set, was the original intent.
I haven't looked into the OutRun example. I'd need you to post, or post a link to, that specific sequence. I suspect it's yet another example of a more modern arrangement, having a polyphony burden that exceeds the SC-55's capabilities.