"STA-OPMODE-SMPS-MODE-CHANGED" what that mean?

Installing and Using OpenWrt Network and Wireless Configuration
21

Hi!
Have the same topic on an GL.iNet B1300 (Qualcomm Atheros IPQ4019 802.11b/g/n).

Sat Jun  3 22:18:42 2023 daemon.notice hostapd: phy0-ap0: STA-OPMODE-SMPS-MODE-CHANGED xx:xx:xx:xx:xx:xx static
Sat Jun  3 22:18:42 2023 daemon.notice hostapd: phy0-ap0: STA-OPMODE-SMPS-MODE-CHANGED xx:xx:xx:xx:xx:xx off
Sat Jun  3 22:18:42 2023 daemon.notice hostapd: phy0-ap0: STA-OPMODE-SMPS-MODE-CHANGED xx:xx:xx:xx:xx:xx static
Sat Jun  3 22:18:42 2023 daemon.notice hostapd: phy0-ap0: STA-OPMODE-SMPS-MODE-CHANGED xx:xx:xx:xx:xx:xx  off
Sat Jun  3 22:18:42 2023 daemon.notice hostapd: phy0-ap0: STA-OPMODE-SMPS-MODE-CHANGED xx:xx:xx:xx:xx:xx  static

All versions I've tried show the same behaviour (19, 21, 22, snapshot). It looks to me like the 2.4 GHz wifi gets quite slow and then completely vanishes (OpenWrt shows it still running, but it stops spreading an ssid). Client is an Android phone. The 5 GHz ssid seems to NOT have this issue! Does anybody have an idea what's wrong here respectively how I can fix this?
Thanks in advance
KR