DIAGNOSIS The printer is receiving a network wake-up command. This is usually from its HP+ firmware or a computer/phone on the network checking for printer status.
FIX Disable the automatic power management. On the printer's touchscreen, go to 'Settings' > 'Prgerät-Einstellungen' (or 'Printer Settings') > 'Auto-Off'. Set the timer to 'Off' or to its maximum delay. Also, turn off 'Auto-On' if the setting is present.
IF THAT DIDN'T WORK It's likely an HP+ or HP Smart feature. If you enrolled in HP+, the printer periodically phones home. The only way to stop this is to leave the printer in 'Sleep' mode, which is less responsive, or disconnect it from Wi-Fi when not in use.
HEADS UP This is normal behavior for connected HP+ printers and is not a defect. It can drain ink slightly faster due to occasional maintenance cycles.