When using pick block to select grass from the environment, if you have a fern on your hot bar it will select the fern every time.

When using pick block to select a fern from the environment, if you have grass in your hot bar and no fern, pick block will select the grass every time.

If you have neither items in your hot bar, it works as expected.

I found this bug in my BDS world while playing on my IOS device, I then tested on a creative super flat on my IOS device and had exactly the same issue. This is on 1.17.41.