Issue with Kpartx

Hi, I think I have found a bug. (Image 2.6.0)
command builds all boards (Pynq Z1/Z2, zcu104) but fails on custom board.
So, I have created new boarddir, reboot system and ran:
“make BOARDDIR=/home/bartek/Eclypse20201/PYNQ/myboards”
As you can see

And after some time:

Don’t know why but have increased index by 1!
Maybe it is caused by mounting cache? for now, as workaround I will set manually boot_dev and root_dev variable in this script