It was revealed by the BBC on their Podlitical podcast that Ms Dugdale, who was in charge of the party during a turbulent two-year period between 2015 and 2017, did not vote for the party at the European Elections held in May.
The broadcaster reported that the former Lothians MSP, who has since taken up a job at the John Smith Centre at Glasgow University, terminated her party membership in the summer, having previously clashed with Labour leader Jeremy Corbyn.
Ms Dugdale, who controversially appeared on the reality TV show "I'm a Celebrity, Get me out of Here," was also unhappy at her treatment by the party in her legal battle with pro-independence blogger Stuart Campbell, editor of 'Wings over Scotland'.
She said the party had 'dropped her like a sack of potatoes' over the case, which she eventually won.
Ms Dugdale previously described the Labour Party as 'sick', and told a magazine after stepping down from Holyrood: “There’s something very fundamentally wrong at that level. If the roots are rotten and not right, you can’t expect the rest of the team to flourish.”
She was succeeded as Scottish Labour leader by Richard Leonard, who is seen as close to UK-wide leader Jeremy Corbyn.
Former parliamentary aide Ms Dugdale, who has previously said it was 'not inconceivable' that she would back independence at a future referendum, publicly backed Owen Smith in his unsuccessful leadership challenge against Mr Corbyn.
Their relationship was understood to have been damaged by the endorsement, with further splits also emerging over the party's Brexit stance.