when a blockstate json file that uses multipart,
have reference to models both with ambient occlusion and without:

the ambient occlusion of any model applied by that file is that of the first model referenced.

for example:

to add a short grass texture on top of some grass blocks,
a grass_block blockstate file is created referencing these 4 block models:

• void: (empty model, ambientocclusion: false)
• grass_short: (short grass above block, ambientocclusion: false)
• grass_block: (vanilla block model, ambientocclusion: true)
• grass_block_snow: (vanilla block model, ambientocclusion: true)

using this json code:

{{ "multipart": [ { "when": {"snowy": "false"}, "apply": [ { "model": "block/void" }, { "model": "block/grass_short" } ] }, { "when": {"snowy": "false"}, "apply": { "model": "block/grass_block" } }, { "when": {"snowy": "true"}, "apply": { "model": "block/grass_block_snow" } } ]}

this renders the desired result of 50% of non-snowy grass blocks having short grass on top,

but any model rendered using this, including the snowy model "inherits" the "ambientocclusion":"false" setting of the void model.

or at least that what I believe happens after testing different configs, and orderings of models / cases.