Note: This is a partial answer(merely a workaround).

Some users have posted about the bug (saying this bug is present from long time)
in reddit and oneplus forums. Since it is a bug, there is no definitive solution to this.

One possible work around is to simply turn off the screen by pressing lock/power button
and turn back on by pressing lock/power button again, and enter your pattern.

Note: This is a workaround.

Some users have posted about the bug (saying this bug is present from long time)
in reddit and oneplus forums. Since it is a bug, there is no definitive solution to this.

One possible work around is to simply turn off the screen by pressing   lock button
and turn back on by pressing   lock button  again, and enter your pattern.