You could try leaving the bathroom extractor fan on all the time (assuming it’s in the false ceiling) to encourage air to flow out rather than in. A more expensive option is to get a one way vent to cover the actual vent hole however suggest sealing it up first for a couple of days to check if that is actually where the smell is coming in.
If you have false ceilings and central air it’s likely there are multiple vents to a common building vent. If it’s a newer building your neighbours are either not using an extractor fan when cooking or it’s incorrectly plumbed.