I changed the cable and also the device. The issue is still there.
I reported the issue to Dell Support, and they informed me that it has been escalated to the engineering team. It remains uncertain when and if the new BIOS will be released to solve this issue.
I noticed that the option in the BIOS only affects the right Thunderbolt 4 port. In contrast, the left Thunderbolt 5 ports work perfectly at a 15W output, regardless of the mode set in the BIOS.
@Easa @SvenC, if your PM18P is still with you, could you test it on your device? It would help in navigating the issue.
The mode switch is visible under the power sub-menu after turning on the advanced setup.
Edit:
I'm not quite sure what causes this issue. When I long-press the power button for 30 seconds while the device is in power-off status, the issue can be temporarily resolved, but the fix is only temporary.
System setup - 280W power adapter and 130W WD19TB (Dual power maybe the issue?)