'Position Control' mode is not set, instead 'Hold' mode is set when using two mode switches


Help me please!

The problem is that I cannot change flight mode of quadcopter to ‘Position Control’ mode but instead ‘Hold’ mode is set when using two mode switches of RC transmitter.
I used Pixhawk2.1. firmware v1.7.3.

But it worked well in firmware v1.5.0 of Pixhawk1.

As I manipulate the transmitter switches to ‘Position Control’ mode, although the highlighted mode in the ‘Flight Mode’ page of QGroundControl is ‘Position Control’, but the actual flight mode displayed in the screen is ‘Hold’ mode and the quadcopter flights in hold mode actually.

What should I check?

Added: When I tested, the problem happens in PX4 from v1.6.0-rc2 to v.1.6.0-rc4 and v1.6.2, v1.6.5 and v1.7.3 But it does not happen in PX4 v1.5.0, v1.5.5 and v1.6.0rc1.

So the problem happens from PX4 v1.6.0-rc2.

Because this problem seems to be a bug, I made a new issue below.