Unhighlighted only removes part of the highlight

[ahmad-b-naser]

I highlight the text, select it again and unhighlight it, but some letters remain highlighted.
(p.s. selecting those letters and unhighlighting doesn't have any effect)
(p.s. If I select a bigger text that includes the highlighted remains and highlight it and unhighlight it again, the old highlighted remains go, however, new remains appear somewhere else)