It seems that the master branch is providing an older version of the ath10k firmware:
When I browse upstream QCA9984 repo, there are several newer revisions of the firmware with 3.10 being the latest numerically. I am curious why the 3.5.3 version was chosen. I cannot find much documentation about this, thanks!
The ath10k-firmware source package is almost unused alltogether (look at the bottom of the file, only two binary packages are still built/ not commented out). After switching to ath10k-ct as default firmware provider, OpenWrt didn't want to maintain the dedicated ath10k-firmware (upstream) package anymore (different firmware ABIs, suboptimal versioning) and now relies on the blobs maintained on kernel.org in the generic linux-firmware package.