You are seeing this because this is a question that was only asked of a single “sample-line” person in each household in 1950. Any others will have it coded as 0. You should be using the sample-line weight SLWT to analyze this variable in the 1950 sample. When you do that you’ll see the zeroes are in line with other years. In the 1940 census, there were also sample line persons, but this question was not a “sample line” question (it was asked from everyone). This page lists the sample line questions in the 1940 and 1950 censuses.