For the motorcycle that doesn't have spark plug wires per say, and was behaving similarly, the issue was a coil-on-plug. Fortunately the motorcycle has a superb built-in test that allows you to individually read and actuate sensors and components.
Since I could individually fire any one of the coils with the push of a dash button, I only had to use a spark plug gap tester to compare the distance at which each plug wouldn't fire. Three of them had similar max distances. One plug had a much, much shorter gap at which it wouldn't fire. That was my bad coil. Not much later a second coil failed. So I replaced them all, even though they're expensive, because they're not easy to get to.
I'd look to find if there's some way you can test and compare the gap lengths for all plugs, and see which one if any, is anomalous.
Remember that the gaps will be quite large because they're not firing under atmospheric compression. The gap may even be larger than your gap tester will support. Make sure you don't zap yourself. It'll really hurt.