Assembling-Machine type entities do not respect energy_source:buffer_capacity

[Blackclaws]

So I've been trying to build an assembly machine that is charged like an accumulator and then slowly consumes the stored energy over a multitude of builds.

However it seems that entities of type assembling-machine do _not_ respect setting
energy_source = {
buffer_capacity = "5MJ"
}

The buffer size seems to be wholly determined by energy_usage.

Specifically this happens with electric energy sources, however I'm guessing others are also affected.

Is this a wontfix or a deliberate choice for specific reasons? Is there any useful workaround available?