This statement is not quite true.
According to this commit:
Remove RPS/XPS support from netifd core, move the logic to a hotplug script that uses a different policy... bla bla bla
https://github.com/openwrt/openwrt/commit/916e33fa1e14b97daf8c9bf07a1b36f9767db679
packet steering was broken in 18.06 and later migrated to 19.07 in the same state.
You can examine the source code of referred script to see the cause of the problem.
As a workaround, many users remove or disable the '/etc/hotplug.d/net/20-smp-tune' script and then add several commands to the init script:
https://forum.openwrt.org/t/r7800-cache-scaling-issue/44187/18
Or:
https://bugs.openwrt.org/index.php?do=details&task_id=2573
echo 3 > /sys/class/net/eth0/queues/rx-0/rps_cpus
echo 3 > /sys/class/net/eth0/queues/tx-0/xps_cpus
Feel free to search the forum.
You will find many topics related to the question "How to restore normal (original) operation of RPS / XPS on 19.07".