I've been dealing with this issue for over a year now. Whenever Num Lock is turned off I'm unable to sneak walk (Sneak then walk forward) as it will instantly put me back into walking normally. It always as well makes it a struggle to "sneak downwards" when flying and move in any horizontal direction. It only does this for the shift keys, if I swap Jump and Sneak, it'll be the same thing except I can't fly upwards.

This can be especially frustrating when building in creative or if I'm doing something like parkour. All 9 of the Numpad numbers keys will cause these issues whenever the numlock is off.

I am a left-handed user with a laptop keyboard where WASD and IJKL make my right arm uncomfortable when using those control schemes. And due to how my laptop's keyboard was designed, the regular arrow keys aren't comfortable to use either since I can't reach any other key easily. This forces me to use the Numpad arrow keys as my movement controls since it is the most comfortable control set for my arms and wrists to use.
 

You can see in the image provided, my normal controls for Minecraft. Since three of my control keys (highlighted in blue and two of which are some of my most used keys) are right next to the num locks key which means I'll be accidentally turning it off and on constantly when I'm not looking at the keyboard. Even when typing in chat. I'll sometimes press it instead of the backspace.

(Just to be clear and repeat myself, it is not just RShift, it does the same thing for LShift. While Numlock is turned off, the issues will occur whenever the Shift key is held down, and then any of the 9 Numpad numbers is also pressed/held down. However, it doesn't seem to affect Ctrl or Enter key in any way.)