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I have sought assistance from three distinct BMW service centres in an attempt to rectify the issue with my vehicle, although they have been unable to identify the problem. As an electrician by profession, I find myself at a loss about further steps to be taken in resolving this matter.

My recommended course of action would be to adopt a passive approach and refrain from taking any action until a significant event occurs.

A temperature of 120 degrees Fahrenheit is considered high for a diesel engine that does not generate much waste heat. In such a scenario, one can question the accuracy of the temperature measurement due to potential errors or other factors.
Do you own a non-contact infrared thermometer to empirically verify temperatures upon obtaining a reading of 120 degrees Fahrenheit?

I have conducted a prior examination, which revealed that the water temperature does not accurately reach 120 degrees, and there is an absence of pressure in the coolant tank.

In such a scenario, it is likely that the problem lies inside the wiring, DME (Digital Motor Electronics), or the gauge itself, since the observed issue does not pertain to real overheating.
Disregard the aforementioned suggestion with utmost effectiveness.

The measurement of car coolant gauges is not entirely precise, and there are instances when they may exhibit significant increases.

As an example, the temperature gauge indicates a reading of 75/80 degrees Fahrenheit, but the needle on the dashboard points to 90 degrees Fahrenheit. According to the Institute for Applied Physics (Inpa), the recorded temperature is 92 degrees. However, there are instances when the temperature rapidly increases to a range of 100-110 degrees.

Did you choose for a lower-cost option when purchasing the coolant temperature sensor? Purchasing original equipment manufacturer (OEM) components from the primary dealer is consistently advisable, since it is one of the few instances when I would consistently prioritise obtaining parts from the main dealer. On the other hand, it is worth noting that an intermittent instrument cluster malfunction is a known occurrence. To troubleshoot the issue, it is recommended to disconnect the battery overnight and then reconnect it in the morning to see whether this action resolves the problem. Occasionally, the most effective approaches are those that are straightforward and uncomplicated.

One of my vehicles had a malfunction in the oil temperature, as it consistently displayed a value of zero despite the installation of a new original equipment manufacturer (OEM) Hella sensor. Additionally, the instrument cluster became stuck in a peculiar cycle and exhibited a refusal to function properly. The needle first rests at the midpoint, thereafter descends to the minimum value, then abruptly ascends to a peak before returning to the midpoint, and finally returns to the minimum value. After removing the battery, the situation has improved without any more issues.

I agree with the previous comment.
The original equipment manufacturer (OEM) component, along with the magic reset, involves the disconnection of the battery for a duration of 20 minutes.