My result: 18.5 to 24.9. "Healthy Weight".
I tried adding 10kgs and it said "Overweight".
Seems ok to me.
They even warn that BMI should be used along with other indicators.
Mechanick, J. (2013). “What If” Being Overweight was Good For You? Endocrine Practice, 19(1), 166–168.
https://doi.org/10.4158/endp.19.1.50042678317gx698
The evidence that being overweight is healthier is a bit dubious, but the evidence that having a "normal" BMI is healthier than "overweight" is nonexistent.