This seems to be a general issue in the snapshot, it is pretty rare that obsidian generates.
Note, that this is unrelated to either of the fixes of MC-93468 or MC-120709, since I was able to reproduce this issue generating a world in 17w49b. So it is something, that broke in the recent snapshots, but is unrelated to any of the changes of 17w50a.

A pretty interesting case is that spot in seed 6507043972746672517 at -165 11 841 where water from the same source created an obsidian block at one spot, but did not at another spot.