It was reported quite early on that a fair amount of tickets was sold (or given, in some cases) to locals who then failed to show up for the match.
A problem particular to Qatar is that most of those locals are very well off, so they have little interest in re-selling tickets they don't use themselves, which means that said tickets end up being completely wasted.
There was a piece on this in The Athletic, including comments from a Somali journo (but then again Somali journos are, of course, notoriously racist/anti-Muslim, so).