First I have to admit I'm not an expert on device encryption. I cannot speak out of first-hand experience, but at least can give some background information which definitely will prove helpful here.
So for one, there's the big confusion about storage naming. Our storage tag-wiki can be a big help on this. In your specific case it's the question what "internal storage" refers to, and how that differs from "internal SDCard" (which it does). So let's see how our internal-storage tag-wiki defines internal storage:
Internal storage generally refers to the storage space on your device that can be used for installing applications and their associated data.
I see our storage tag-wiki is even more precise/detailed:
- device storage: usually referred to as "internal storage" or "phone storage". […] On your device, this will be used for apps and data, and usually is mounted at
- internal SDCard: some devices offer a separate internal SDCard, where you can store your own data (such as documents, videos, music files, pictures, etc.).
So from this you can clearly see the difference. Without root, you're not able to (freely) store your personal data (as defined above) on device storage. But our apps have their own specific directories here where they can (and do) save their settings, data, and more.
Second confusion is caused by different manufacturers using different "mount points" and access-paths for storages. As you describe for your device, they might even mount the SDCard inside the
/data partition. However, if the encryption setup offers you separate choices to encrypt internal storage and internal SDCard, this should not matter: If you can tell it to not encrypt a given partition, one should assume it follows this advice.
If, on the other hand, you don't get the option to not touch the internal SDCard, I'm not sure it will be left out. In that case, it might very much depend on your device (and system). When switching from UMS to MTP to provide access to your "internal SDCard" from your computer, for many of them there was a second change going along: to not "fragment" the internal storage space (i.e. "internal storage" running low while there's plenty of space available on "internal SD", or vice versa), they just use a single partition to hold both. The "internal SDCard" then is created as FUSE device, in simplified terms: pointing to a directory on internal storage. If that's true for your device, encryption of "internal storage" most likely will affect the "internal SDCard" as well. But as initially stated: I have no first-hand experience here, and thus gladly accept corrections in the comments.