Again, what gasoline are you guys using? The newer more efficient engines are more finicky than the older ones, and when BMW says 91 (US/Canada) or 95-96 (Europe), they really mean it.
A problem is that 91 is unavailable in large parts of the US, and mixing 89 and 93 is a hassle.
While you can certainly drive it on 89, the engine will adjust by delaying the spark, which gives worse fuel economy and somewhat reduced power output. So if you have been filling with 89, worse fuel economy is expected. If you've used 91 as recommended, the reason is elsewhere.