Interesting finding of PRD on HP-81

[AndiGer]

Don't know anybody realised already:
The HP-81 has 20 registers. Addressed thru 0 - 9 and .0 to .9
I filled registers 1 to 9 with its own number, register 0 with 10, the secondary registers .1 to .9 with 11 thru 19 and .0 with 20.
When I perform a PRD (print DATA) registers 1 thru 9 and .0 thru .9 and their contents are printed but not register 0 and its content. RCL 0 prints 10.
Is it intentional or a (known) bug? Don't know as I don't own the owner's manual. I have a handbook for HP-80 that has only 1 register - and of course no printer.
Andi