Yeah, this is a known issue. This is due to our anti-cheat plugin. It's not ideal, but it does prevent people from using fly hacks and the like. From one SCB lover to another, I HATE this plugin. I can't tell you how many times I've died from glitched jumps. Unfortunately, people will cheat if we remove it, so our choices are the occasional glitched jump or an invasion of people using fly hacks.
The best way to prevent a glitched jump is by not initiating a double-jump while sprinting. Sprinting makes your jump go farther and makes our anti-cheat plugin more likely to think you're cheating. If you stop running/moving, your jumps are much less likely to fail. I hope this helps!