I'll write what happened in steps as it occurred (the problem starts at steps 4-6, if you want to skip to that):

1. After waking up I noticed my phone was only at 50-something % after charging for 6 hours overnight from 30%. The USB tester it was connected through read constant 1.36A (around 7W). No abnormal heating observed.
2. Assuming this was kept constant, the phone should have received 42Wh overnight. I rebooted the phone, tried charging it turned off, percentage remained the same. "Whatever you say then..."
3. I tried different adapter (Xiaomi). Near-15W for a few seconds, then back to 1.36A. I often see this exact figure, thus I assume the phone learns my charging habits and slows down. I tried changing the time from around 04:30 to 12:30, but this made no difference.
4. I tried the official USB-C PD 66W adapter. Unfortunately, I do not have a USB-C tester, but Ampere app reported >5A and the percentage was indeed going up quickly.
5. I connected it back to the previous adapter, finally getting near-15W at 5V as anticipated.
6. I just put a small fan back on top of the screen (not necessary, but it was on that 66W adapter for a bit before - the cooler the better) as usual and was about to leave it charging, but soon heard a strange notification sound. Upon unlocking I was greeted with the error shown.
7. Glancing at the USB tester, it was erratically jumping between 5V, 6V and 12V, protocol jumping between "Unknown" and QC 2.0.
8. I reconnected it with the same results.
9. I switched to Xiaomi power adapter (the suspicious one being Motorola), no issues observed at first. I assumed power adapter failure.
10. I shortly connected the phone back to the official 66W adapter to asses potential damage. Everything worked.
11. I connected the phone back to the Xiaomi adapter without a USB tester, and once again got the error shown (this time on a different QC-compatible adapter). Re-connecting with a USB tester the issue didn't show up again.

Devices list

Phone

Ulefone Armor 24 (version shipped with Android 14) - 85.14Wh

USB Tester

FNIRSI FNB18

Cables

1. HOCO USB-A (4-pin*) to USB-C data + charging
2. Choetech 240W USB-C to USB-C 2m

Adapters

1. Motorola TurboPower 15W (USB-A)
2. Xiaomi TurboCharge 33W (USB-A*)
3. Ulefone 66W PD (USB-C) - (11V 6A for 66W)

* Regarding the 4-pin USB-A: Xiaomi breaks standards by utilizing USB Power Delivery via 5-pin USB-A with their proprietary cable.

Thanks for any ideas.