Here's the full Brothers log:
2020: 1-1 (game 2 102-99 win vs. Raptors; game 6 125-122 loss vs. Raptors)
2019: 1-1 (game 2 home win vs. Indy, 99-91; game 2 road loss at Milwaukee, 123-103
2018: 0-2 (game 6 road loss at Milwaukee 97-86; game 4 road loss at Philly [up 3-0 in series) 103-92)
2017: 0-2 (game 6 102-101 road loss at Washington; game 2 130-86 home loss vs. Cavs)
2016: 0-2 (game 2 89-72 road loss at Atlanta; game 6 104-92 home loss vs. Atlanta)
2015: 0-1 (game 4 101-93 home loss vs. Cavs)
2012: 2-1 (game 4 101-79 home win vs. Atlanta; game 7 85-75 home win vs. Sixers; game 6 home loss vs. Heat 98-79)
2011: 0-1 (game 4 98-90 home loss vs. Heat)
2008: 2-0 (game 5 96-89 home win vs. Cavs; game 6 89-81 win at Detroit)
2003: 0-1 (game 2 89-77 loss at Pacers)
So that's 6 wins, so the 3-13 stat I don't think is even correct...
6-12 so far, nothing crazy out of the ordinary. So far this seems more circumstantial than anything. Last year they got crushed all series after game 1. In 2018, pair of road losses. 2017, a close road loss in a series the Celts won (so that one may jump out a bit, although quick glance showed 19 fouls on the Celts vs 24 on the Wiz), then a blowout at home to the superior Cavs. In 2016 they lost the series 4-2 to Atlanta, Brothers did 2 of those games, potentially curious. And in 2015 the Celts were swept.
I'm going to keep adding to this. So far the losses are pretty much all road blowout losses