Russia definitely tried to interfere in the 2019 election - the trouble is that they seemed to do so in the wrong direction, trying to help out the anti-Brexit side: https://www.bbc.co.uk/news/uk-politics-53433523 (People spotted this quite a while ago but the government hasn't commented on it previously.)
It's not clear that Labour were on "the anti-Brexit side":
"If the Labour Party win the General Election they want to renegotiate Prime Minister Boris Johnson’s Brexit deal and put it to another public vote alongside the option of Remain."[0]
Perhaps if you think that the public would have voted to Remain, then that's an anti-Brexit policy, but perhaps Putin just liked the idea of the chaos that such a referendum would bring (and possibly the supposed benefit of having Corbyn as PM).
