C# windows form: checkbox in the first row looks unmarked

I am using Datagridview for showing some data. I added a column of a checkbox, and when the grid loads I set that all the checkbox will be marked (set the value to true).

But I had a strange problem. The scenario was:

1) Open form at the first time, all the checkboxes are marked well.

2) Selecting another row (that not the first row)

3) minimizing form.

4) maximizing form - and then all the rows are marked, besides the first row.

I checked the real value of the checkbox in the first row, and that value was true! but the checkbox is still looking unchecked. If I'm selecting another row, the checkbox returns to be marked

Does anyone have an idea what could be the problem? Thanks.