The debris-in-the-brake-or-sidepod-duct thing happens every season since 1950 to one or more cars.
One of the many question marks about this car is: why did the team revert to their 2017-2019 sidepod design (sidepods with undercut and little downwash) and ditched their 2020 Rb-style sidepods, after only one season, although the 2020 car improved a lot with that design...In addition to that, every team in the grid now has adopted that RB style design or incorporated its features to their own design, (except Alfa and Haas - but both teams are special cases)It could be that the 2020 sidepods, the Renault version of those sidepods*, have reached their development limits and arent very suitable to the 2021 smaller floor...So maybe the team was kinda caught between a rock and a hard place regarding the 2020 sidepod design, and because of the 2021 rules, so they opted for more undercut...
*The Renault version is a bit 'différent'/less slender because some features of their PU and the packaging prevents the Enstone team from having an aggressive downwash shape and from tapering the top and the sides of their sidepod bodywork (aft the radiator).Firstly it's because of the classic shape and the width of their exhaust manifold, which is partly dictated by their non-split turbo layout. So Renault have to extend the bodywork further back than others to have a smooth transition around the manifold...So latter plays a big role in this area.Nextgen-auto - Cataluña CircuitBesides the more cumbersome manifolds, the RS/Alpine sidepods need to fit the compressor-to-interccoler pipe (green here) unlike the others, and Enstone somehow makes them pass underneath the manifold, on the floor...And that also prevents you from slimming the bodywork and the coke bottle area, on paper
Regarding the brake ducts, RS.20 rear ducts were noticeably small. The 2021 are bigger and are roughly back to their 2019 shape and sizeSo A521 is on the leftPics from motorsport.com
Looking back, I could also "conclude" that Renault PU is too chunky at exactly where the "Mercedes side pod concept" needs to have its body work slimed down. Thereby the limit is met.
But what you wrote, all teams adopt RB side pods,... if so, the reason is plain simple, because it makes the car goes faster.
Hence, Alpine need to work on sliming down the PU and its ancillary to get the level of Honda... ouch...