Some years ago I implemented a Sudoku checker and solver on Windows XP/Qt 4.8.1/Visual Studio 8. Recently I reactivated the code on Xindows 8/Qt 5.13.1/Visual Studio 2017. The application is working fine but a strange effect I already noticed on the old system is still there.

The main Sudoku panel, meaning the fields for the 81 digits I realized using 81 QLineEdit with setMaxLength(1) and setInputMask("D"). I can display loaded or calculated Sudokus and I can also input digits when defining or solving ones.

The strange thing I would like to understand and failed to find in documentation or other threads happens when I do a mouse click to select the QLineEdit where I want to input a digit.
There are two sensitive points in the QLineEdit although MaxLength==1.
When I click in the center of the field I get a cursur in the center that takes about a third of the place and no input is accepted.
When I click on the left third of the field I get a cursor in the left third and I have all the functionality I want.

If somebody could explain why there are two separate sensitive points and what I could do to get rid of the unusable one of the two that would be really great.