It's not the "being off-lead" per se, it's the loose dog approaching the trapped dog unchecked. I have absolutely no issue with a loose dog passing us with plenty of room and without even glancing over, that's just fine...zero eye contact and a distance of at least 150 yards is perfect. She'll still plunge and scream at the proximity of another dog, which sometimes attracts them for a closer look.
For perspective, my dogs recall is excellent. She rarely goes far, she does if she gets a scent (working terrier) but always comes back when called. So poor recall is not why she's on a lead - she's on a lead because she's very aggressive and will attack other dogs unprovoked, large or small. So when a loose dog comes over, she attacks in fear, then collapses in terror and gets savaged by the retaliating dog whilst trapped on her lead. She's been attacked 7 times.
You could anthropomorphise and say it serves her right, and I can't deny that she certainly starts it - but she's frightened and that's why she's on a lead and muzzled, so if your loose dog rushes over to play, gets muzzle-punched in the face and instinctively attacks her, that becomes YOUR problem. Mine was under control before you fucked it all up by not keeping yours away.
I just walk when and where we never have to see another dog or owner, but even in places where dogs are supposed to be on a lead she's been hurt by loose dogs, because obviously other owners are mysteriously exempt from the rules. I hope her spine gives out and we have to let her go soon, it'll be a relief for both of us.
A dog up our road is always getting out, it attacked my dog through our fence and ripped her face open. I loathe other people.