The following highlights the problem:

    auto img = QImage(1, 272, QImage::Format_Indexed8);
    auto foo = img.convertToFormat(QImage::Format_RGB16);
    qWarning() << foo.format() << foo.depth(); // Format_RGB16, depth==32!

I think the conversion to RGB16 effectively converts to RGB32 and then in-place to RGB16.

I guess that the issue was introduced in b10e1209e1c3655ee6e57185bb5375408ce02e56. It is not present in 5.14.0.