Hi there and welcome!
Often
Masterbuilt digital eletric smokers (MES) can never even hit their stated max of 275F temperature so if you have one that actually hits 300F degrees I think you are in way better shape then most hahhaha.
The MES smokers are notorious for having temp probes that are just plane off. As much as 15-30F degrees in some cases. So all of us electric guys also get dual probe (or more probes) wireless digital thermometers and we test the thermometer probes to know how accurate they are. They should read 212F degrees in boiling water or close to it.
Next we clip those probes at meat level on the rack in the smoker and that gives us the most accurate temp reading to go by. You can then adjust your MES temp up or down according to your wireless thermometer rack readings. I would suggest u get a thermometer with 4 -6 probes but 2 probes as the minimum, there are a number of good ones out there.
The screws you see the "button" looking sensor are fastening the safety rollout limit switch to the smoker. That "button" sensor will cut power off to the heating element for overtemp safety reasons, if it reads a smoker temp of 301F degrees.
The other sensor that sticks out is the probe that the MES controller is using to read temp and is the one that is reporting 300F to the display.
Knowing how notoriously bad all MES temp probes are (smoker and meat probes) I imagine you arent really hitting 300F, but only getting a multiprobe wireless thermometer will solve your mystery.
Don't worry. We all get these multi probe wireless thermometers and yes they do cost a pretty penny BUT they are a necessity if you want to control your bbq smoking with precision and accuracy and to know what is really going on in your smoker. It just sucks that
Masterbuilt skimps so hard in this area BUT the rest of the physical smoker is where they really shine :)
I hope this info helps :)