Jon also fueled rumors he might have met Rachel in person before cameras started rolling and got her pregnant when he posted one of his own baby photos, showing a striking resemblance to sweet little Lucy.
"That baby looks like she could be yours. Y'all are so adorable together! My fav couple by far," wrote one Instagram user.
"Are you sure Lucy isn't yours? She looks like you," another fan commented.
However, Jon and Rachel -- who were recently shown getting engaged on TLC's 90 Day Fiance: Before the 90 Days -- have insisted on social media Jon is only Lucy's stepfather and father figure.
"Just you and a million other people who like pointing out something which is impossible. She looks like Rachel. Huge brown eyes," Jon, 34, replied to all the chatter.
Rachel, 33, also chimed in with her two cents, showing she's a bit frustrated by all the gossip.
"Good grief... Has this conspiracy not been put to rest yet? She's got big brown eyes like me... Jesus Christ... Let it go," she wrote.
The identity of Lucy's biological father has not been disclosed, but according toIn Touch Weekly, a woman claiming to be Lucy's paternal grandmother attempted to throw Rachel under the bus back in August.
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The woman allegedly accused Rachel of refusing to let Lucy's father be a part of the baby's life. The woman claimed Rachel initially agreed to co-parent Lucy with the man but changed her mind at the last minute.
"Rachel Bear you are one mean and hateful person," the woman reportedly wrote in a lengthy Facebook post.
"You never notified anyone that you had the baby or let anyone be a part of her life, you are a selfish b-st-rd. You robbed her of meeting any of her family. You endangered her and your other daughter... by picking up some guy on the Internet."
Rachel, who is from Albuquerque, New Mexico, and Jon, who is from England, star on Season 2 of 90 Day Fiance: Before the 90 Days, which premiered on August 5 and currently airs on Sunday nights on TLC.
After getting to know each other for a year-and-a-half over the phone and video chat, they finally met in person for the first time in England in December 2017.
So far on the series, Rachel and Jon's relationship has been strong. Not only did Jon propose in England, but the latest episode showed the couple preparing for their wedding.
Rachel decided to overlook Jon's criminal past out of love and believes he's not a cheater although she caught him having a Facebook conversation with an ex-girlfriend in which he admitted he had truly been in love with her and regretted hurting her.