Hi Richard, thanks for the help.

Regarding the drawing...will try to get it and post here.
In the meantime here are the answers:

are the leds connected to gnd or vdd ?
The LEDs are connected to ground


are the leds powered by the same supply?
Yes, same power supply as the MCU

what size is the led series resistor ?
It's 500ohms for a SMD LED

and what does actually happen compared what you expect to happen
The same moment that i turn the power on, the LEDs turn on and then off very fast.
After some seconds (when the code kicks in) the LED animation starts.

I expect the LEDs not flash when turning the power on and only turn on when the code kicks in.