No duplex with HPLIP 3.17.10 and Color LaserJet Pro MFP M377dw

I have a Color LaserJet Pro MFP M377dw, which works great, except for one feature: the "Two-sided" option in application print dialogs is always grayed out and shown as "Not availeble". That's a pity because I know the hardware supports it.

Using Gentoo, HPLIP 3.17.10, installed the printer using hp-setup, and hp-plugin says that the proprietary plugin is installed.

Slightly odd thing is that when I look at the CUPS printer administration at http://localhost:631/printers/HP_Color_LaserJet_MFP_M377dw, the printer driver is shown as "HP Color LaserJet Pro MFP m177fw" always, no matter what I try to install. I am suspecting that this is the problem but I'm not sure which other driver to use since there seems to be none that exactly fits my printer, and this is the one hp-setup wanted to install.

Any pointers for help would be much appreciated! Thanks in advance!