Ask your DM to implement individual initiative in these instances
Group initiative for monsters is convenient, but has limitations. From what you describe in your question, your DM is (probably accidentally) exploiting group initiative for maximum advantage. Taking away five actions with one action is, as others have pointed out, very powerful, but it's less than you're entitled to in this case. Let's compare how this works with individual and grouped initiatives respectively.
Let's say there are six goblins who each roll initiative. Goblin 1 goes, first, then goblin 2, and so on.
Imagine you succeed in putting all but one of these goblins to sleep. There's a 1 in 6 chance that the remaining goblin is goblin 1, who acts first. He wakes goblin 2, who then wakes goblin 3 and so on, until all the goblins are awake by the end of the round.
The other five times out of six, that isn't what happens: if the goblin who evaded the spell was one of goblins 2-6, they wouldn't be able to wake all of the other goblins within a round. Furthermore, one might think it unusual if the Goblins were uncannily aware of the turn order, and utilised this knowledge for maximum wake-ups.*
The goblins all act 'simultaneously'. This is not quite true, however, as their turns occur in a sequence arbitrarily (but not maliciously) determined by the DM. This way, when one goblin remains awake, they are free to wake another goblin, who in turn wakes another until (as you have described) all the goblins are awake within a round. The trouble with this approach is that your DM has, probably without their own knowledge, arbitrarily decided that the goblin who evaded was the first goblin to act, even if that goblin would not have acted first otherwise. What would, using individual initiatives, be an unfortunate coincidence instead becomes the only possibility.
Talk to your DM
Sleep can be a frustrating spell to balance around. It becomes less powerful at higher levels, but at tier 1 it can wipe away entire encounters. Your DM is probably doing their best to create encounters that are engaging and challenging, and they would benefit from your feedback.
Ask your DM to use individual initiative. This doesn't have to be all the time, but randomly assigning turn order within groups on-the-fly would be enough to give you the utility that the Sleep spell entitles you to.
It's worth noting that your DM is currently using an optional rule from the Dungeon Master's Guide (DMG) known as side initiative, in which each side takes a turn and the creatures on that side act in any order they choose. Helpfully, the DMG introduces its chapter on optional rules with the advice that, before adding such a rule, the DM should ask:
- Will the rule improve my game?
- Will my players like it?
Your DM is not breaking the rules or adjudicating badly, but they might need your help to create a more mutually enjoyable experience in this instance.
* This depends on your game philosophy; if players can take advantage of their knowledge of the turn order, why not Goblins? It's secondary to this answer.