I've got the Masterbox 5 Lite RGB too, just arrived yesterday, and same issue. If I remove the front panel the temps drop about 9 degrees C cooler on my CPU and 7 degrees C cooler on my M2 SSD that sits next to it, and GPU is down about 5 degrees C. These are all idle temps too.
When I install the front panel cover, temps rise within about 3-5 minutes. It's really, really, really poor design with the extremely limited front intake space and the front shroud sitting so close to the case fans leaving no breathing room. This is all with the front fans running at full speed, because although CM remembered to include an RGB splitter, the apparent focus on form over function made them think it was okay to include extremely short fan cables and think it okay to have them run off molex power thus permanently at 100%. The ever present hum of maxed fans which provide terrible airflow is going to drive me nuts.
I'm definitely thinking of returning the case and going with something from Fractal or Corsair instead.
CM should be shipping out replacement front panels that are larger in dimension and allow for proper airflow and at least a 3pin fan splitter that allows the fans to spin up/down via motherboard (or fan controller/hub) to customers who thought they were buying a higher tiered product.
I would forgo RGB and tempered glass panels to get a case which is quieter under desktop operation and actually allows airflow, although the competition manages to offer many/most/all of these things no problem.