Need advice on Dexters recent behaviour

Hi everyone, I'm hoping that you can give me some advice on Dexters behaviour and how to correct it. He's 18 months old and always been a happy, energetic puppy but very friendly (to other dogs and people). Other dog walkers have often commented that both Dexter and Milo are very well behaved (as much as they can be for their age), which I've been very proud of but his behaviour towards some other dogs has changed over the last couple of weeks.
For a bit of background, we lived in a very country area until recently (military housing around lots of countryside), so when they went for walks, did not often meet many other dogs to socialise with. Now that we've moved to the city they've been walking in a local park everyday, which always has plenty of other dogs to play with. At first they were both a bit unsure of other dogs but now they happily play with other dogs, no problem.

In the last couple of weeks, there's been a couple of instances where Dexter has trotted up to other dogs, had the usual sniff, but instead of then bounding around them to play (as he usually does), he's snarled & snapped at them. This has been pretty confusing and embarrassing for me, as I'm super paranoid about any aggressive behaviour with them being staffs and having such a bad rep. He never used to do this, he's always been happy with other dogs (off the lead, they've both been funny when approached closely by other dogs, on the lead, another issue I'm trying to deal with).
So does anyone have any ideas what could be causing this? Or how to stop it? I can't think of any external factors that could be the issue. There's been no change at home in the last few weeks, he's getting the same amount of exercise, I did change his food two weeks ago but they wouldn't affect him like this would it? It's not a case of me being nervous because I've not changed my behaviour when walking him. I'm going to keep him on the lead for now, around other dogs until I know what to do. I don't think this is a permanent solution though. Any ideas?

It could be that he just doesn't like the dogs he snapped at. Suki is great with most dogs but there are some she just doesn't like. I would just keep doing what you are doing and make sure to supervise the initial interaction.

I'm just worried cause a lot of people are wary of them anyway due to their breed, then if their dog does come over and Dexter goes for them, they'll assume that he's a dangerous/aggressive dog and always like that. I don't want to stop him from socialising with all dogs though. Also it doesn't help that he gets funny when dogs approach him whilst he's on a lead. So if he's off his lead and snaps at another dog, they'll think I'm being irresponsible and he should be on a lead but if he's on his lead he'll probably go for them anyway, as I think he feels trapped
I'm feeling really nervous about it now because of an incident on Sunday. He'd been playing happily with various other dogs, throughout the walk, then he went over to a small puppy (who was on a lead). They gave each other a little sniff, then Dexter went for him. He didn't hurt the dog but sort of snarled and snapped at him. The owner picked his dog up, I ran over and he proceeded to scream and shout at me to 'get it on a lead!!' and that if I can't control them, then I shouldn't have them. I can understand that he was upset because his dog was shook up (being quite young) and it was Dexters fault but the guy just went crazy! Now I'm doubting myself, thinking that I must've done something wrong. I wont let him go to another dog who's on a lead now but he did snap at a dog the next day, when neither of them were on leads. Just don't really know what to do now