It looks like the FW on QCA9984 already reports the tx airtimes before
the station is added to the peer entry. The peer entry is created in
ath10k_peer_map_event() just with the vdev_id and the ethaddr, but
not with a station entry, this is added later in ath10k_peer_create() in
callbacks from mac80211.
When there is no sta added to the peer entry, this function fails
because it calls ieee80211_sta_register_airtime() with NULL.
This was reported in OpenWrt some time ago:
https://bugs.openwrt.org/index.php?do=details&task_id=2414