Oh, that strong sensation of deja vu....I recall several threads speculating about what BTC might do pre/post-halvening, and IIRC members who thought that it was priced in were shot down (or at least there wasn't much support for that theory).
I happen to think all of the halvings are priced in, and that's if there's anything to price in at all. There's no reason why bitcoin should do anything when the block reward gets chopped in half, right? Just because the miners need a higher price to keep them in the black doesn't provide an impetus for anything at all to happen to the market.
And I remember writing a few times that no matter what my opinion is, bitcoin could rise based on people speculating that the halving is going to boost the price, and I think that's happened. Ah, I don't know. I'm just going to watch like I always do and root for $100k--but I'm hoping it gets there gradually, not in a massive 2017 spike.
This probably describes best what's going on. Coming up with a technical explanation for the BTC price to rise or not around the halving isn't very helpful on an individual basis. It still remains pure speculation, but the psychological tendency of the majority of people is probably the assumption that the price still rises. The perceived increased scarcity when the block reward drops is likely to have a bigger impact on the price than the idea that the scarcity concept is already priced in and people may take profits around the halving.
It is the same with expected and actual inflation and how expected inflation drives actual inflation. It often needs time and only via a moving average can realistic conclusions be drawn where people really thought the inflation rate would end up being.
BTC's price could be overpriced, or underpriced, but the interesting thing about expectations in the BTC network is that halvings are hard-coded. Inflation isn't hard-coded and can depend on a million of things, like a pandemic, logistic problems, war, trade wars, climate, etc. One could assume that expectations could be priced in easier for the BTC network since increased scarcity isn't the result of unexpected events, and yet it is a hard nut to crack.
I believe it is more about a narrative when it comes to halvings. They have manifested themselves as being BTC price booster, but technically they shouldn't be as everyone can look up the schedule and then make up their minds about a potential future price tag. The truth is that BTC reflects far, far, far more of the complex world than just a pre-defined halving schedule / programmed emission curve / scarcity. But the halving schedule is the easiest to understand of all the things that BTC interacts with. It's something people can easily follow via a countdown. However, complex developments like geopolitical tensions or aspects from political economy aren't something that many people can reasonably evaluate. That's why I think these halvings, despite being perfectly predictable, can still set free some of the known herding behavior.