my sneak/crouch keeps glitching

first of all, I've searched quite a while on youtube and here and I have found nothing 

second, I don't know if this is an actual bug or if there is a button I can press to stop this but I have tried everything and it just won't fix it.

alright, so basically every time I press shift it works but sometimes when I am sprinting while pressing shift or just walking normally it glitches. What I mean by 'glitches' is that it makes my character stand up really quickly and then crouch again. This is really stressful when I'm trying to build a bridge and my character suddenly glitches and I fall off. Please fix this thank you  

extra information: I have tried version 1.8.2, 1.8.1 and 1.14  

also, in the video I am not letting go of sneak/crouch button on purpose, that's the glitch.