I asked my son the following question: the Shulchan Aruch (O.C. 10) writes that if one has 4 cornered garment that is slit open exactly half way, there is a safeik whether it is chayav in tzitzis or not (the garment must be mostly open to create corners that are vaday chayav) and therefore it should not be worn outside on Shabbos. The M”B quotes the MG”A as saying that not only should it not be worn in a reshus harabim, but it shouldn’t be worn even in a karmelis lest the tzitzis not be necessary and be considered carrying.
Why not say that since carrying in a karmelis is only an issur derabbanan, you should be allowed to put yourself in the situation of sfeika derabbanan l’kula and wear the garment?
There is, of course, an obvious distinction: the MG”A is speaking about an issur; Tosfos is speaking about a kiyum mitzvah. But why should that distinction make a difference? If you can put yourself in a situation of safeik, who cares if the root of the safeik is a kiyum mitzvah or a safeik issur?