During testing I had directly connected my Wemos D1 ProMini via the built-in micro-USB connector (i.e. 5V converted to 3V3 via D1's built-in regulator) and the device started up properly most of the time.
In my "prod" setup I am powering the device via a 12V to 5V (1A) buck converter that I am connecting directly to the D1's 5V pin and I now found, that the device ISN'T starting properly most of the time upon power-up (I had expected that the D1 would behave the same as if connected to the USB's 5V).
But it does then start propery when I press the reset some seconds after it has been powered up. (Once everything is nicely warmed up it then usually does start correctly right from the start.)
I guess that the problem might be caused by some voltage fluctuations during initial cold start - but I don't know enough about electronics so I don't really know what is going on.
Is there anything I can add to my circuit to make the thing startup properly every time?