I'm not sure how they managed this but some of my players made the lifts with open sides and move out of the intended path of the lift.

After this they somehow get stuck in fly mode.

using lift v12 and the new recommended CB 2126?

Actually its not just escapee's it sometimes just happens. I just used a lift several times between 3 floors and after using it for about 20-30 times over a few hours I suddenly was in creative flying mode.

Somewhere the plugin forgets to set the user.canfly = false after using the lift.

Should be fixed in version 13