Hi,
to the problem:
The first I´d check is PO.3 signal and PO.4 signal.
You need to do this with a scope and try to find any capacitive coupling.
You surely know that a triac just needs a short pulse to switch ON.
So even a short pulse on the port pins may cause a problem.
(I´ve noticed your extra capacitor on the gate ... to make it less sensitve on glitches)
After this you know whether the problem is on the microcontroller side or the mains side.
The next possible problem is PCB layout.
the signal LINE needs some kind of star point wiring (to avoid problematic voltage drop).
The star point should be at pin 2 of the triac ... for each channel.
In other words: The trace from Triac_pin_2 to according capacitor C1 (C3) should not see any load current. This way you guarantee that the C1_LINE voltage is the same as triac_pin2 voltage.
Depending on the inner circuit of the LED bulb (did you check if it is designed for phase control?) there will be a high inrush current. This inrush current may shift the voltage levels on your PCB (if not routed correctly)
Please upload your PCB layout. (Mind a useful file size)
***
BTW: I dont understand the waste of bandwidth. Why is your schematic that huge?
This (the expanded! if you click on the preview) is less than 1% of your filesize. Thus it loads much faster .. especially in regions with low internet bandwidth.
Klaus