There is a widespread belief in a simple explanation among Googlers: device makers test against iPhone. Whenever Apple strays off spec, they'll choose to be compatible with Apple's implementation rather than the standard.

TBF, can't blame them. Apple has pretty much a single stack. Android is about as fragmented as the PC world.

That wasn't a common belief AFAIK among people I talked with that actually knew what they were talking about. They readily admitted the Android stack sucked balls. There are 3 generations of Bluetooth stack that I recall. The first was Bluez-based but offloaded a lot to the vendor as I recall. That didn't work well. The second one Broadcom open-sourced their BlueDroid stack and donated it to Android in 2012. The third is Google finally taking control of it and I hope they had success with it (my info is a bit dated on that front).

Google has had a single Bluetooth stack for quite a while although variable quality vendor implementation. IIRC BlueDroid took over more of the stack than Bluez did so we're talking about 10 years here where Android has had a single monolithic stack.

The main explanation I've heard is that Apple has much fewer Bluetooth chips to support but I don't know that explains it because there's generally not that many chips in the market that all phones choose from (granted Apple is now in-house but still, they didn't use to be).

Well I think it doesn't stop with the SW stack, I remember the time when the android devices had the bathtub notch without having any real sensors in them.