Disick, 37, and Richie, 21, had also been quarantining separately, which reportedly took a toll on their relationship.
The source also told Page Six that Disick's relationship with ex Kourtney Kardashian, the mother of Disick's three kids, weighed on Richie.
"Kourtney didn't make it easy for her, and you can only tolerate that kind of treatment for so long".
Disick and Richie began dating in 2017. They split briefly in 2018 after reports emerged that Disick had cheated on her, but they later reconciled.
Earlier this month Disick reportedly checked into rehab for alcohol and cocaine abuse.
Insiders told DailyMailTV the 36-year-old reality star has been struggling with family life during lockdown, and is still having a hard time coping with the death of his own parents a few years ago.