Robby had a big smile on his face and flashed a smug look when he agreed with Amanda that things didn't work out because they both have different expectations of what being in a relationship entails.
Shortly after the finale aired, Robby explained his side of the story to fans and critics on Twitter.
"While I won't be one to resort to vicious social media attacks, at the finale, I felt like I was sitting next to a stranger. I'll never understand why there's such a desperate need to represent an 'innocent persona' and resort to allowing allegations that are blatantly untrue be said about me by 'the twins' while one sits back and 'unknowingly' watches these malicious attacks continue against me," Robby tweeted without naming Amanda directly.
"That's not the behavior or accountability I expect from someone that 'CLAIMS' they care about me and was open to developing an 'ACTUAL' relationship. I've upheld their names both publicly and privately with respect, regardless of the several ongoing THREATS I received."
Robby then addressed claims he cheated on Amanda with a girl at a Zac Brown concert in Colorado. Robby called the accusations "false" during the finale and insisted that he flew to Colorado on "business" and was photographed "being close to a girl" but not making out with her.
"Yes, I'm friendly and personable with many people, but I have respect for myself and even the person I'm casually dating, so I keep my interactions appropriate. I don't use the media or my friends to handle personal relationship misunderstandings, as we just saw them do, and have seen happen many times before," Robby continued in his rant.
"History is repeatedly showing that some people thrive off of using the media and their 'army' to manipulate viewers to place one-sided blame, constantly play the 'victim' and use their friends to try and assassinate one's character. Your story-line is getting OLD; at some point it's just on YOU to stop living a double-life, and at this point, everyone sees through it. God bless."
About 15 minutes after Robby shared this hostile post, Amanda shared one of the photos on Twitter allegedly proving that Robby had been unfaithful to her.
In the picture, a blonde girl with a ponytail is leaning into Robby's shoulder and face, and they appear to be kissing or at least showing a little public display of affection for each other.
Amanda captioned image, "Look what you made me do," which is a quote from Taylor Swift's new hit song.
After one Twitter user argued the photo doesn't show evidence of kissing, Amanda replied, "I have plenty more [plus] videos. Plus multiple girls have came out and DM'd me with stories since...etc etc."
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Prior to posting the photo, Amanda also tweeted "'A desperate need to present an innocent persona.' - the guy who denies cheating when there is photo proof #beamanalready."
Robby and Amanda, a single mom of two daughters, appeared to end things on amicable terms on last night's finale, but clearly that wasn't the case.
Prior to dating on Bachelor in Paradise 4 this summer, Robby previously competed for JoJo Fletcher on The Bachelorette's twelfth season and finished as her runner-up. He was also reportedly briefly linked to Sarah Vendal, Amanda's good friend.
Amanda first appeared on Ben Higgins' season of The Bachelor and then got engaged to Robby's friend Josh Murray on last summer's Bachelor inParadise edition.
Amanda and Josh had a tumultuous relationship and nasty split in December 2016.