Jersey sales only apply to players who have played enough games to make the rankings. Meaning if your player plays every league game, it won't matter if at mid or late season, 3 or 4 of your opponents WO in 1-2 consecutive weeks. They will still get jersey sales. It is not a bug, but rather there is a loophole in the system.
Do you know why you didn't get any jersey sales? Look at your league page, at the league leaders part. It says "Minimum 1 games played" on the right side of the box. That number grows as the season goes on. By the end of the season, it should be 15 games. So it means that if your player played 15 games in a row, then all your opponents give you WOs for the rest of the season, you will still get jersey sales. The only time you won't, is like I mentioned, is if your player is one game short of the number needed to qualify for the rankings (or if they have enough games for that week but the number goes up the next week, then 2 WO in a row would give him 0 jersey sales). Hence it is most likely to happen at the start of the season. Chances of it happening later on is really rare unless you play your players minimally.
What happened is that your player was recorded to play 0 games due to the walkover (I assume this is done so as not to affect the game average stats), so that's why it doesn't qualify for jersey sales. So it's definitely not a bug.
I think what we need is walkovers to be counted as games played in the "minimum games needed to qualify for rankings", but not counted in the average stats calculations.