Buffy is already in her Betrayal Outfit as of the end of #36... it's what she changed into after her shower in Spike's cabin. Since she's not wearing that outfit in the "Soon..." panel with Spike, that panel must take place after the betrayal.
Assuming that the artist and editor are as anal as we are...
Good point! I really doubt Spike is gonna be the one who betrays her anyway 😉