Beam depth restrictions work like this. If the user assigns a depth restriction that is too limiting and no beam within the depth restriction works, then the program provides a warning in the Design warnings log something like " Beam X on Level Y, Unable to optimize for depth restriction given.", then it proceeds to select a beam outside of that limit. Beams with warnings are also highlighted on screen using the View - Show beams with warnings command.
When the output is generated for beam design, such a beam will be listed as "(Optimum)", and this optimum size will be used for self weight, material takes offs, exports to ISM etc.
Thus for any beam that you don't want such behavior it's better to either make the beam Other material or you can use Assign - Beam size and select the very last shape called "No Design".