Nijntje91 the reason is only you. You dig and dig until you find a bone. And because it is software, you call that bone a bug.
This nice picture explains everything (more or less). Your airplane is just a number of bits in the computer. Some of these bits are used for calculation AND for graphical representation. That is if you pull your stick, you get an updated calculation and an updated graphical representation. Other bits are only used for calculation. The elevator trim does not show in the graphical representation, but you get an updated calculation.
This is not completly correct. In non-cockpit view I have graphical representation of elevator trim.
See file:///C:/MSFS%20SDK/Documentation/03-Content_Configuration/SimObjects/00-Aircraft/Flight_Model.html#what-is-a-flight-dynamics-model