It's not a "50" thing. Checking the first 80 members:
Numbers 1, 2, 5, 10, 12, 38 - 42, 44 - 46, 48, 51, 53 - 68, 70 - 72, 74 - 80 have good join dates. Although some of them in the 40s show August as the month they joined. It should be July.
Numbers 3, 4, 6 - 9, 11, 13 - 37, 43, 47, 50, 52, 69, 73 have screwed up join dates.
Numbers 1, 2, 5, 10, 12, 38 - 42, 44 - 46, 48, 51, 53 - 68, 70 - 72, 74 - 80 have good join dates. Although some of them in the 40s show August as the month they joined. It should be July.
Numbers 3, 4, 6 - 9, 11, 13 - 37, 43, 47, 50, 52, 69, 73 have screwed up join dates.