Thank you very much for your comment and questions. So here we go:
Depending on the health organization/source referred to, there are varying degrees and divisions of the BMI. To keep it as easy to understand as possible, I have not listed so many subdivisions, but gave ranges for which they in general agree. The range from 20-25 is generally accepted as "normal" and the range from 18.5 to 20 is in some sources referred to as "slightly underweight". (1, 2)
So if you have a BMI indicatin a "sligthly underweight" state many physicians would view you as still fine, but you should try not to lose more weight or maybe even gain some.
In general being "slightly underweight" from the point of view of a Western lifestyle is considered as something quite beneficial for longevity. (3)
Nonetheless there are some links between being slim or even underweight to the development of some diseases. E.g. Chronically inflammatory bowel disease statistically more often affects thin than obese people. But it is not yet clear whether the underweight favors the disease or whether both are just symptoms of another predisposition. Finally, a clear indication of the relationship between general mortality and the BMI.

Source 3

I hope I answered your question!

Best,
mountain.phil28