What’s causing this pattern?

Looks like a very small amount of pillowing (search here for that word):

http://support.3dverkstan.se/article/23-a-visual-ultimaker-troubleshooting-guide

 

Is the second picture the bottom of the print?

 

pillowing and what looks like a not so perfect first layer

 

With respect to the bottom layer, the voids can be the result of insufficient adhesion or slight warp in the bed or printing too fast or needing a small increase in layer one overlap. It could be a combination of all any combination or all of the above.

Personally I have never seen a flat (non-warped) build plate. The exception are plates that are milled to be flat, but I don't know of a consumer 3D printer that uses a milled build plate.

Things that can be done to improve the first layer could be to slightly increase the layer one extrusion multiplier or use MESH or UBL leveling schemes rather than LINEAR which Robo seem reluctant to move away from or slow layer one print speeds or increase the percentage of layer one extrusion overlap.

tldr; Respecting leveling, Robo uses LINEAR and despite my explaining to them directly how LINEAR works they don't want to change it to a better scheme. Even BILINEAR would be better, although only marginally. LINEAR takes into account all probe points and then creates a flat plane that best fits all the points at once. It is one plane, so some points will be above the plane and some will be below the plane. BILINEAR bisects the leveling plane at every point that is probed so instead of a single plane it is multiple planes like a checkerboard, this fits a warped bed much better. The planes created with LINEAR and BILINEAR are single use only and are not saved, hence the need to re-level for every print. MESH and UBL use the BILINEAR method but also can be saved forever in EEPROM. UBL has the advantage that it can extrapolate any bed warp from the outer probe points to the edges of the build platform.

 

@supercazzola You can use my newer version of Marlin as it is preconfigured for UBL. One caveat is that you won't be able to use the Z Offset wizard because of poor programming logic in RoboLCD. I posted a bug report on their GITHUB but Robo appears to not want to improve the interface instead wasting time on stripping Marlin code they feel is unnecessary.