Text Widget Code/Content Missing After Upgrade

After upgrading to 4.8, WordPress seems to be stripping out any manually added code/text in my sidebars. Any pre-built widgets (Recent Posts) emerge unscathed, though it doesn’t recognize the custom text in the header.

So for example, if you look at the source HTML, you’ll see the divs for the widget but instead of code in between, it’s a few blank characters.

I have tried this with the new Text widget, the plugin Classic Text Widget, and the plugin Visual Editor (BlackStudio). In each case, the manually input text or code does not get recognized.

I know there have been a number of weird bugs following 4.8. Anyone else have something similar? I am running W3 Cache, but I tried deactivating it with no success. My host may have a cache on their hardware as well, I’m not sure.