DBGrid - lose last edited field (last visible on screen field is always NULL after edition)

Paradox 7 / Delphi 3.0

I use TDBGrid to edit simply databse table.
I do not want to edit all the fields , so I created grid just with 3 of

When I edit the fields, I lose the last edited (visible in grid) field.

When I check the fields after button 'post' of TDBNavigator is pressed, I
can see that 2 first fields have proper (just entered) values, third field
is simply 'Null' - entered value disappeared ! When I attach additional
column to my grid, value entered in 3rd field is kept, but 4th field is

Summary: when I use DBGrid last (visible on screen) field is NULL after
What is going on ? Where is the reason? (I am new in 'Delphi' area)

Please help.

Thank you, Marek.