How to find the source of a wakelock when it is unknown?

Several days ago I noticed that my phone (Redmi 4 Prime) is consuming battery more than usual when it is not being used (over night for example). After checking the dumpsys batterystats I see a huge number of these in the log:

+13m00s059ms (2) 074 +running wake_reason=0:"unknown"
+13m00s350ms (1) 074 -running
+13m03s137ms (2) 074 +running wake_reason=0:"unknown"
+13m03s421ms (1) 074 -running
+13m06s211ms (2) 074 +running wake_reason=0:"unknown"
+13m06s495ms (1) 074 -running
+13m09s277ms (2) 074 +running wake_reason=0:"unknown"
+13m09s562ms (1) 074 -running
+13m12s351ms (2) 074 +running wake_reason=0:"unknown"
+13m12s634ms (1) 074 -running
+13m15s116ms (2) 074 +running wake_reason=0:"unknown"
....

All wakeup reasons:
Wakeup reason unknown: 1h 11m 38s 198ms (14188 times) realtime

So I started with a clean slate; Did a full wipe and installed a custom rom without Google apps (just wireless and mobile data was active) and the strange thing is that as soon as the screen of the phone is off, those unknown wakeups appear again in the batterystats log!!!

I tried with multiple custom roms (Havoc-os P, CrDroid Q, Mdroid 8.1, …) and all show this strange behavior.

What tool can I use to find the source of the wakeups?

Since this behavior shows up even on newly-flashed roms with no apps (even Google apps), can it be the case that my wifi access point is causing it?