QDoubleSpinBox maximum rounds incorrectly when working with big numbers

When using QDoubleSpinBox maximum with big numbers, the program at runtime rounds the max incorrectly. The tests were made on a widget project on a Windows machine using QT 5.8. The values were set with the Qt Designer - not in code. The values look OK in the Designer but during runtime they round unexpectedly. The issues were found only in QDoubleSpinBox, QSpinBox behaves as expected.

The other variables in the QDoubleSpinBox do not seem to matter.

Examples:

When entering 999 999 as max the program behaves normally -> the max is 999 999

999 999.99 rounds up to 1 000 000.00

999 999.50 rounds up to 1 000 000.00

999 999.49 rounds down to 999 999.00

9 999 999 rounds up to 10 000 000

9 999 995 rounds up to 10 000 000

9 999 994 rounds down to 9 999 990

99 999 999 rounds up to 100 000 000

99 999 950 rounds up to 100 000 000

99 999 949 rounds down to 99 999 900

and so on...