I'm seeing the same symptoms (with a "Qualcomm Atheros QCA6174 802.11ac Wireless Network Adapter (rev 32)"), but it started with me on buster after upgrading linux-image from 2.11.x to 2.12.x (2.13.x also has the same issue). It looks like 2.12 bumped the firmware API version used for this chipset from 5 to 6, and there's a new firmware-6.bin (& board-2.bin) available upstream (
https://git.kernel.org/pub/scm/linux/kernel/git/firmware/linux-firmware.git/).
I grabbed the ath10k/QCA6174 directory (commit 4ebfab3 is the latest change there) and that fixed the issue for me. Looks like those were last updated on 20170918, which is newer than the firmware-nonfree/firmware-atheros package.