The TAB key or call robot-mode-indent-line works incorrect

[badger1961]

If I load a robot file and try to indent a line using TAB key or explicitly call robot-mode-indent-line, Iine is indent using one space instead of 4 space as it defined in variable robot-mode-basic-offset. (By default this variable is equal 4 spaces.)
From other hands if I have text

MY_TEST_CASE
     keyword1
     keyword2
     keyword3

I will have

MY_TEST_CASE
 keyword1
 keyword2
 keyword3

Could you please look and help me?
I use emacs 27.1 on kubuntu 20.10

WBR
Alexander