Q. Is that seuda considered (a seuda on the fiftieth yortzait – see prior question) a seudas mitzva?

A. As mentioned on the last question, it is difficult to find a source for a special “fiftieth year seuda” minhag.
However, Horav Shlomo Miller’s Shlit’a opinion is that if the minhag of that family or congregation is to make a seuda with a siyum or other divrei Torah on any yortzait (as is the case of many offering a tikun after davening), it does become a seudas mitzva.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit’a