Unknown charging problems

I've started experiencing charging issues on Latitude 7390 not so long ago.

The symptoms:

- blinking LED on an AC adapter

- upon powering on, charging speeds fluctuate in range of 0 and 30W

- severe throttling issues with AC plugged in (but not otherwise)

- unplugging AC at <20% charge results in instant power loss

- If battery is discharged, it may not start charging for hours of plugged-in time.

Known:

- probably not an adapter issue: got a replacement, exactly the same problem

- probably not a battery issue: same reasoning

- probably not a DC input connector issue: same reasoning

- tried CMOS reset, hard-reset (residual power draining)

Technical:

- Model: Latitude 7390

- CPU: Intel Core i5 8350U 1.7-3.6Ghz

- BIOS: Latitude 7290/7390/7490 System BIOS v1.4.1

- OS in use: Debian 12 Bookworm GNU/Linux, kernel version 6.11.2