[REG 5.6.2 -> 5.6.3] QSyntaxHighlighter rehighlight broken after upgrading from Qt 5.6.2

I have a desktop app used across several platforms written in Qt/QML. Below is description of one of the things which got broken after upgrading to Qt 5.9.3.

I have TextEdit with text which is being checked for spelling mistakes. This is implemented using QSyntaxHighlighter which highlights errors in red (created in Component.onCompleted of the TextEdit). Internal Spellchecker in my app emits signals which are connected to rehighlight() of the highlighter. Everything used to work like a charm in Qt 5.6.2 but when I upgraded to Qt 5.6.3 this just got broken - rehighlighting never takes place. Downgrade made it work again.