Personally, I think our supposed problems against that teams that defend low are a bit overblown, since Bruno arrived. This is our PL record in games that fit this description during that period:
WBA 1-0
CHE 0-0
NEW 4-1
CP 1-3
WHU 1-1
SH 2-2
CP 2-0
AV 3-0
BRI 3-0
BOU 5-2
SHU 3-0
WAT 3-0
8 wins, 3 draws, 1 loss. 28 goals in 12 games. 9 goals against. 7 clean sheets. At least 3 goals scored in 7 of those 12 games. The one loss was the first game of this season, with all the particular factors impacting that (lack of pre-season etc). 1 of the 3 draws were against Chelsea, whom anyone would struggle to score against when they come to play as defensively as they did in that game.
In other words, more than half of the time, we blow the opposition away (3 goals or more). And we've won 2/3 of the games. This doesn't look to me like a record suggesting a serious systemic problem against teams that defend low.
If there's a negative, it's that the results were much more positive during the first 6 games than in the last 6, which has yielded a result of 2 wins, 3 draws and 1 loss, with 8 goals scored, 7 conceded and 2 clean sheets. Only one game with 3 goals or more scored. Which is of course why this issue is again being raised. However, we still have the same players (and more besides) and the same formation that produced those results last spring, and it's plain from the results that recent struggles can't be due to our not having "cracked the code" on how to beat low-defending teams, or that we lack the personnel or formation to do so.
One notable factor seems to me to be that we've generally been far more successful when we've fielded our best players up front (Rashford, Bruno, Martial, Greenwood) than when we haven't. Which we haven't in any of the above PL games so far this season, for various reasons. This may suggest that as long as we do that, we have the capability to beat low-defending teams, but we don't have the squad depth to perpetuate that ability when we resort to secondary options. Another notable aspect that is hard to miss is that the marvelous run against weak teams last season coincided with Pogbas re-entry into the team. Not saying he was the cause, but there may at least be something about the CM needing more of an offensive spark than McTominay/Fred can provide in such games.