Here’s a small example of why there are sometimes problems with cutting through plywood. It is 4mm “laser suitable” BB plywood.
The last image is the settings I “normally” use for this material.
…it is mm/min I always use, but there was not enough space on the disk 
Because you are stating mm/min I’m thinking diode laser. I use an 80W CO2, 4mm is not much of a problem. I do get the same scorch marks on the top and the bottom from the bed. I use masking tape on the top and bottom now and scorch marks are greatly reduced.
No, I have a K40 with approx. 35 Watt, but I started with a diode laser and also use it occasionally.
Setting mm / min, personally gives me significantly more nuance in my speed settings.
Usually I rarely use tape, the results are fine as they are but sometimes I sand the surface with very fine sandpaper, depending on what I make. In my example, however, discoloration is extreme because I “will with all my might” through the plywood and used several passes. The underside of the items is usually the nice side, it is probably related to my needle bed and a relatively good extraction.
Looks like a void that had been filled in the plywood. I use a very powerful light to look for voids and unknown filler spots to try and avoid the area.
Yes it can be annoying with those kinds of surprises. If it does not happen too often and not in a large and complicated part, I just make a new one. One’s items also lose stability at these inclusions and if it happens in a bearing part, of my lamp constructions for example - then I have a problem.
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The only real ‘laser’ ply is two veneers covering a thin MDF core. The rest is marketing BS.
Voids, glue amount and type, core plies type and quality all have an effect on repeatability.
The worst ply I’ve used was marketed as Baltic Birch - but it had a poplar core which burned like a torch with the least effort, fanned inside the plies by the air assist.
For expensive products I’ve been using 2.5mm mdf with a veneer, that I buy from a local cabinet-maker. He lays the veneer to order, but does ask that I buy a handful of full sheets, as it’s a process to set up.
The difference in repeatability of that over any plywood I’ve used is marked. Always the same, nice clean tan cuts, no scorching, even if it’s a little slower cutting MDF due to the density.
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I also use a lot of MDF and HDF which have the properties you describe. I actually think it is also a nice material without being wrapped in plywood. I use it primarily for boxes and mechanical parts like gears.
You can actually buy some BB plywood they call airplane plywood (interior and exterior) which is a real high quality product. But on the other hand, it is not financially suitable for my type of production.
Plywood with soft wood inside is something the devil has invented to tease us laser people - you have hit the nail precisely.
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