Broland,
I don't think it's a bug at all. I could be wrong, but I'm seeing too many patterns among the "missing businesses" to think it's not intentional. I think 2 things are happening right now based on what I've seen.
1. I believe Google is A/B testing at the moment which explains the crazy fluctuation we're still seeing daily on trackers like
Algoroo
2. I believe the criteria for the local search results filter has changed and is now filtering businesses in the same category in the same building. I have a client in NYC who is popping in and out of the 3-pack and every time he disappears, the guy in his building who offers the same service appears and vice versa.
I have a few more observations I'm consistently seeing that changed on the filter as well that I'll be putting together an article on hopefully next week.