The plank abutting the house does look different, and the gap width between it and second plank inconsistent in width. My decks were redone in 2012 and involved all planks. Perhaps the variable gap took up any inconsistency where the new planking finished.
In my research years ago I discovered TDS products which are specific to deck work. I also discovered the need to have the caulk not attach at the bottom of the "groove". But, as you need to rework a small area you could use Sikaflex (?291, they make several versions),readily available at Bunnings. Remove the old decaying caulk with a fine scraper like a narrow bent tip screwdriver, tape the edges,and fill. You might need to sand afterwards to flat.
I remember searching online and finding videos of how to do it,and pages of illustrated instructions. But you might not need to overthink what I hope is a small job. Though it could recur as other areas fail.
2005 Integrity 386 "Sojourn"