Start with an article with an inline alien - e.g. "ABC<tt>:-)</tt>DEF". Place cursor "up page" (in LTR, left) of inline alien; selection with keyboard down-page works as expected. Now place the cursor "down page" (in LTR, right) of the inline alien; selection with keyboard up-page fails when reaching the alien, clearing the selection and placing the cursor immediately up-page of the alien.
Note that in RTL this appears to work fine(!).
Confirmed in both Chrome and Firefox.