In some cases Language Tool does not recognize “haven’t” as an auxiliary verb and suggests to replace a past participle with a past tense.
To reproduce type either of those lines (with quotes):
"Haven't you seen it?" “Haven't you seen it?”
to the form on the front page, select any English language and press “Check Text”. Language Tool will suggest to replace “seen” with “saw”.
Same in LibreOffice.
Works correctly without quotes or in non-interrogative direct speech.