Had a chance to try this now to my Nothing (which is allegedly 33W capable). I'm using a GMM 120W charger and typically get 24W as shown on a cheapo USB cable-with-display. Insert the FNB58 into the chain and after an initial restart when the phone is attached, it goes as expected: climbs to 24W and stays there.
Then I unplug the phone and the FNB58 show no current but 9V still. Plug the phone in and I get about 3W for a short while, then the FNB58 reboots and comes back up showing 5V, 10W. After a few seconds, the voltage drops to 4.7V and then the FNB58 reboots and repeats the 5V, 10W, reboot thing. Seemingly forever.
If I disconnect and reconnect the charge the same thing happens (the FNB58 sucks power from the phone in that case). The only way out is to disconnect everything and start again from scratch.
I tried connecting a second power source to the micro-B (PC) connector, and that prevented the initial reboot-on-connect, but on disconnecting and reconnecting the phone it went back into the 5V, 10W, reboot loop.
With the FNB58 out of circuit, no problem: phone always ends up at 24W.
Is this normal behaviour for the FNB58 or do I have a borked one?
[Edit: added video]