This may be related to T147827: Cursoring across focusable nodes has problems in Safari 10.
Result: It skipped from "Sentence." to "Next" without selecting either ref.
I tested this with https://de.wikipedia.org/wiki/Browser_Extensions#Kompatibilit.C3.A4t refs 9 and 10 in Firefox, and it works almost as expected for me. Only when going from left to right, I need to press the right arrow key twice between the two references, going from right to left only needs one keypress.
But I am able to select the two references and placing the cursor between them.
For me, it took a couple of key presses to get between things, but it never highlighted either ref.
The behaviour here is very inconsistent. It seems to alternate between working fine, then taking extra key presses to do anything, then skipping some of the references, then skipping them all. The problem is intermittent.
This does not seem to be breaking anything at present.