Apparently, this is a bug that for some unknown reason has not been fixed until now. If you write open_buymenu in the console and do not move, then invulnerability will be extended by 10 seconds each time you enter a command. And entering bind X open_buymenu will keep you invulnerable and on the move. In this case, you need to press the X button. True, the very first shot "removes" invulnerability.

It is not yet reported whether the developers are aware of the problem, as well as when they plan to fix the problem. However, for sure many will take advantage of the situation.