Expansive Spellstrike allows for using spells that can "target a creature or that [have] an area of a burst, cone, or line (abiding by any other restrictions of Spellstrike)" with Spellstrike. It has this caveat on spells with multiple targets: "If the spell lets you select a number of targets, it instead targets only the creature you attacked with your Strike."
Chain Lightning targets "1 creature, plus any number of additional creatures". Clearly it can be used with Expansive Spellstrike, but do I get to actually hit multiple creatures with Chain Lightning? On one hand, it seems like I should be able to chain the lightning, so long as I start with the first creature I Spellstrike, since the spell functionally seems to target one creature and then have additional possible targets (dependent on the creature's save). But on the other hand, it is a spell that targets multiple creatures, so maybe I'm limited to just hitting the first?