Is the pinyin for 他撇了撇嘴 actually "tā piē le piězuǐ" (as given by Pleco)? And if so, why are the two 撇s pronounced differently?
撇 is pronounced as piē when it means 'to cast away' (as in 撇下兒女) or 'to scoop off' (as in 撇油). Note it is a verb in both cases.

撇 is pronounced as piě when it means 'left-downward stroke' (as a noun in 撇捺), 'to throw (horizontally)' (as in 撇磚頭), or 'to incline' (outwards, as in 八字腳向外撇). 撇嘴 literally means to incline the ends of the lips downwards, usually to express discontent; so it must be pronounced in the third tone.

I suspect Pleco detected the first 撇 as a verb, then jumped to the conclusion it should be pronounced in the first tone. It failed to realise 撇 here is a detached verb and is collocated with 嘴. After tone sandhi 撇了撇嘴 should be pronounced like 3023.



Reference: 余培英（2012）《多音字學習手冊》。北京：商務。