If you [...] have advantage on an attack roll against one of the targets, you can forgo the advantage for that roll to make an additional weapon attack against that target ...
So, let's say I am invisible as per the greater invisibility spell, or the enemy I'm attacking is blinded, or something else; either way, constant advantage on all my attacks.
At this level, I can make 3 attacks via Extra Attack, so let's say I hit an enemy twice, with advantage, but then for my 3rd attack, I decide to forgo my advantage for an additional weapon attack, as per the Rapid Strike class feature.
Clearly this 3rd attack will not be made at advantage because I deliberately chose to forgo advantage as per Rapid Strike. But would the additional (4th) attack I make still be made at advantage? Or is it implicit that this additional attack is also forgoing advantage?