iserith
Magic Wordsmith
Whether it says that in 5e somewhere or not, I have no idea. It probably comes from an earlier edition, and the goblins in my world haven't changed, even if the MM or Volo's Guide or whatever decides to make them different.
Sure, I point this out merely to show that seeing goblins as cowardly isn't universal or even firmly grounded in D&D 5e lore. Though truthfully I haven't read the section in Volo's on goblins, so it might very well say something different. Either way, I think my point stands.
Cowardly or not, though, I was thinking about it, and it's an interesting question regardless. You're in the midst of battle, are put to sleep by a spell, and then are rudely awakened when you're hit by a mace. A quick look around the battlefield shows your opponents all standing, and your allies all lying unconscious or dead. The nearest escape takes you past your fallen allies but to where additional allies are waiting, so what do you do?
Whether a goblin would figure out they are asleep due to the same spell is probably less relevant than whether they would attempt to defend/save an ally, and what their version of ally is compared to ours. If it's a survival of the fittest sort of thing, they might just very well leave them.
Honestly, I don't bother to think about it this deeply during play. At least not normally. I think of what will be the most fun for everyone and lead to an exciting, memorable tale. I do that. I justify it later by establishing the appropriate justifications, if it's even necessary.
I'd also like to clarify that I don't have any issue with how you adjudicated the scene. Finding a clever escape is always fun. It's just a good example, as you point out, of how the initiative system creates a scenario that probably wouldn't otherwise occur, and also had the effect of more or less negating the sleep spell entirely, also as you pointed out. It's also an example of something that probably wouldn't ever occur in our campaign because we use an alternate combat system that wouldn't have artificially created the circumstance for this to occur.
The start/stop nature of turn-based combat is something we dislike altogether, and the group initiative makes it even more apparent. As others have pointed out, the circumstance was pretty rare. It had to be goblins, and it had to put all of them to sleep (although casting a sleep spell against any number of goblins just before their initiative turn would have the same effect since any of the goblins that are awake can start the chain).
Whether that bothers you or not is really up to you and certainly not wrong, and as written I think it's the right call.
I like to paraphrase a Churchill quote about democracy with regard to standard D&D initiative: It's the worst system... until you consider all the alternatives.