JNP

Then we both have the same LED strips, my printer is also an LDO kit. With a constant current source, the output voltage is regulated to achieve the 350mA. In...

It is clear that the current increases slightly at 24V. At 28V they are even brighter ;-)

If you connect the LED strips in series, they should be brighter.

Do you have an STLink V2? You could use this to check whether the MCU is still accessible via the SWD interface.

Does the 3.3V LED light up when the board is powered?

In 24V operation.