What is the reason that Chabad doesn’t say Barchu after Aleinu in Ma’ariv if one missed it in the beginning?
Perhaps this is due to the opinion that Barchu cannot be recited without a Bracha afterwards. See also here: https://asktherav.com/6749-minhag-chabad-regarding-pores-al-shema/ Sources: ראה בריטב״א הל׳ ברכות ו, א ובחי׳ ריטב״א…




