Backwards tabbing gets trapped in a cycle

Use the provided sample to create an executable that demonstrates the issue:

When tabbing forwards the focus follows the defined order from the designer UI file.

When backwards tabbing (Shift-Tab), the focus is trapped in the both QGroupBoxes (Machine Type and Clock Unit).

It's even more prominent with the FLASH0 tab activated: there the backward tabbing alternates betwerrn two QSpinBoxes ignoring the QCheckBox in the same QGroupBox completely.

The problem is also noticable in 5.15.2. but not in 4.8.7.