I’m pretty sure SSDs can only do 4kib aligned writes regardless of the FS sector size (under the hood it’s a write amplification unless the OS or controller manage to coalesce them. But yea, it depends on how things are getting flushed, but generally I wouldn’t expect too much magic unless you get lucky. It sounds like a small bug in the OS (ie these kinds of wires should be matched in memory in the application).

The logical block size presented by SSDs to the host system is 512 bytes or rarely 4kB. But the native page size of the flash memory itself is usually more like 16kB, and the erase block size is several MB at a minimum. Those larger sizes are why random writes (and especially random overwrites) can cause high write amplification within the SSD: because what looks like a series of single-sector writes to the host will at a minimum cause fragmentation within the SSD, and can easily cause large read-modify-write operations within the SSD.

Normally, SSDs and operating systems both use aggressive caching to combine writes. That's the only way a drive can turn in extremely high random write benchmark numbers. Consumer SSDs do this caching even though they do not have power loss protection capacitors to ensure that data cached in volatile SRAM will be flushed to the flash in an emergency. But it wouldn't be smart for the caching to wait forever for more writes to combine with a sub-page write, which is why I'd be concerned that a slow and steady trickle of write activity may be able to cause serious real write amplification.