The attached C++ example qtbug107543.zip overrides QStandardItemModel::headerData() to return green for the first 2 rows and red for the rest.

Steps to reproduce:

• Build the example in debug mode, note colors appear as expected
• Build the example in release mode, note colors appear randomly different. Typically, only the last row is red

Note: It appears to be fixed in 6.4.1 .

Originally reported against Qt for Python: When subclassing QAbstractTableModel; changing the background color of a header based on a condition sometimes work and sometimes not. 

see attached sample model class that overrides headerData and uses Qt.BackgroundRole to change the color

it was working fine on 6.3.1

update: 

I've updated the attachments to provide a working example along with screenshots of the results

631.png demonstrates the correct outcome using 6.3.1

632.png demonstrates the wrong outcome using 6.3.2