What to do if AirPods Max malfunction after latest macOS 15.4 update?

Have you shut down and restarted all devices more than once since your 15.4...(?)

I do not think the issue is related to the 15.4 update at all. Correlation does not imply causation.

In fact I already experienced a very similar problem with two pairs of AirPods Max (with no audio playing at all). After experiencing this with the second pair I started digging deeper, and in the end solved it for both headphones, even though the first ones hadn't been working for months.

There is a common issue with AirPods, that they experience some form of condensation, which could to be related to the issue. Among other things, this can cause oxidization on the plugs between the headband and the pads. (The headband can be removed with an SIM removal tool, when the pads are turned into the off/carrying position.) Sometimes it's enough to clean these oxidizations off with some deluded alcohol and letting it dry before reassembling.

For me on either set of headphones I did not have any visible oxidization at all, and cleaning did not help either. What fixed it for me was some off brand thin contact spray, which I sprayed directly into the headbands sockets (in extremely tiny amounts). THIS IS NOT RECOMMENDED, however. The sockets do have some very finicky contacts. Deforming those essentially destroy the AirPods Max'. If you choose to try this approach on your own discretion, let the AirPods dry for several hour in a non-humid, warm place after applying the contact spray. In my case this fixed the issues. Every few months I need to repeat this steps, though.