Pattern-avoiding modified ascent sequences

We initiate an in-depth study of pattern avoidance on modified ascent sequences. Our main technique consists in using Stanley's standardization to obtain a transport theorem between primitive modified ascent sequences and permutations avoiding a bivincular pattern of length three. We enumerate some patterns via bijections with other combinatorial structures such as Fishburn permutations, lattice paths and set partitions. We settle the last remaining case of a conjecture by Duncan and Steingr\'imsson by proving that modified ascent sequences avoiding 2321 are counted by the Bell numbers.