Looks like a spot color.
How does it look with View > Overprint Preview?
If your document is in CMYK color mode, any swatches you create that are outside the gamut of that space (which it looks like yours is), they will convert to the closest possible value in your artwork based on your Color settings.
Yes it is CMYK color mode.
RGB looks as it is.
If you are working on a digital design, then file >> Doucment Color Mode >> RGB
If Print then CMYK
If you still have the issue, send us a screnshot of your edit >> color settings
It is designed for printing.
If you use Pantone spot colors your are using colors that can only be printed by mixing special inks to print that color. These are not normal CMYK printing inks and can be previewed by using Overprint preview.
The dull color you see is how the color would look if converted to normal CMYK inks.