I'm running Android 7.1.1, LineageOS 14.1. But for some reason apps can't write to internal storage.
Examples: can't download received Whatsapp images, can't save photos from Facebook, can't download Swiftkey themes, can't take Instagram photos. If I open the stock Gallery app I get a "No external storage available" error, and it quits. I can take photos with Camera to external SD, if I try to select Internal, it tells me there's not enough space. I can however, install apps from the Play Store to Internal.
So something's messed up with permissions, I guess. In Storage Settings the built-in SD is called Internal shared storage.