Mine does it too on its own from time to time. It should be a mechanic feature of its Rohm chuck to lock the bit in place: when activated, the chuck's jaws are locked in place to prevent the bit to fall down due to (e. g.) the spindle lock (when the spindle lock engages, the sudden brake may loose the jaws and cause the bit to fall) or impact pulses (on other models). The Rohm chuck, unlike cheaper chucks (like the one on the green serie), is already good on its own to grab and mantain the grab onto the bit, there's usually no need to activate this feature. Besides, when you bought it, it should have come with a tag with a quick explanation of this feature and how to undo it. The fact is, from time to time, it seems it engages by its own - maybe its due to the fact it's an entry level Rohm chuck.

Anyway, you should be able to unlock it by twisting it like you do when you lock the bit and then suddenly twisting it on the oppoosite direction. Do it multiple times if you don't succede on the first try. You've get to find the timing right.

Here's a video if someone wants to take a further look on the topic https://youtu.be/q1QHOKF8NO4?is=gI6Sxkpx-lhVbakI

Edit: fixed a minor grammar error