"questionable behaviour" is correct and can be used in written English.
It refers to actions or behaviors that are doubtful or uncertain in terms of their morality, appropriateness, or legality. Example: The politician's questionable behaviour has caused public outrage and calls for his resignation.
Where it is a question of others' questionable behaviour, a standard answer is that it is up to each individual to decide for himself.
Not only does their row highlight the questionable behaviour of both men.
Think of settlements or charitable donations made to banish the aftertaste of ethically questionable behaviour.
A legitimate ploy to expose questionable behaviour that could not otherwise be exposed.
And he's having to face up to some slightly questionable behaviour that he exhibited before he got fired.
In the 10 years I've taught at boys private schools, I have seen questionable behaviour in students too.
Among other questionable behaviours, her adviser seethed when she did some work with a rival lab during her adviser's sabbatical.
The newspaper discovered that the ATF regularly set up shady pawnshops and smoke shops in cities across America in order to catch criminals, then engaged in some questionable behaviours that included employing – and then arresting – mentally handicapped individuals, destroying property and losing track of guns.
Our data suggested that in certain situations mental health workers may have demonstrated ethically questionable behaviours when interacting with detained service users.
Conservative leader Andrew Scheer then asked Clement to step down from caucus Wednesday after more allegations about his questionable online behaviour emerged.
Some films depict questionable bee behaviour, like the opening scene in The Exorcism of Emily Rose, where bees are seen travelling in a snowfall.
