It looks like your scanner is scanning with a limited spectrum when you ask to scan BW begative images, hence the flattening when luminance details come from blue and green channels, and little from red channel. When I take the red channel from the color scan you published, I get (with few adjustements with 'curves') a very close result as your BW scans, hence I strongly believe your scan takes only the red spectrum.

In order to get the real result with this model of scanner, I recommend you to scan in color, then to flatten in BW your image. Unless your want to rewrite your scanner drivers (which can be fun), or unless you can tune such parameter as "the spectrum used for BW scan". You can play with color filter, channels specific adjustments to optimize your result, the same way photographers were using orange and red filters with BW films at the shooting moment to soften and lighten skin tones for instance.

I worked a lot with XP2, and I always got the best results from scanning in color. even though my scanner was working with full spectrum in BW mode.

Today, when I want to do BW images, I shoot in color (digital) and then work the color channels later, this gives me maximum flexibility and creativity.

Good luck