Our solution was something I learned this early on and therefore my puppy has been taught this from her first walk and consistently on each walk to date.
Initially when she had any other dog in sight I used the "watch me" command for her to direct her attention to me and I rewarded her.
Now at any point that she see's another dog (at any distance) she looks me in the eye.
This training was easy as our breed has the instinct of handler focus and I was able to draw on her natural strengths.
At first when I begun researching how to navigate this type of reactivity I came across one of the threads of this forum that explains how many of us are the reason for their actions. When WE see another dog, we may tense up in anticipation and begin to bark commands of "leave it! no! sit! stay!" etc., etc., thereby inadvertently rewarding the undesired behaviour.
If she does become reactive it is typically the other dog initiating some threatening or playful behaviour at which point I use the "leave it" command with or without success. With time you'll recognize the other dog (whether it be breed or behaviour) on the leash and you should either redirect the attention onto you if you think it possible or turn around/cross the street.
As with all training, consistent perfect practice is the key to success.
I've never used a muzzle, are you able to easily give your pup treats through it?
Could it be that somehow the muzzle has drawn the curiosity of other dogs to her and she's reacting to their initial behaviour and that is why the 2nd or 3rd meeting is uneventful? To answer this question we would need to know if your dog only has this reaction on a leash or upon first meeting in a different setting.
That's my method and some of my thoughts.
I'm sure you'll get a lot of good advice here, good luck to you both!