FKA twigs, who dated the star from 2014 to 2017, can attest to the extreme nature of these fans. She told BBC Radio 4's Grounded with Louis Theroux that her relationship with Robert made her a target for racism and other attacks.
"People just called me the most hurtful and ignorant and horrible names on the planet," the singer shared. "It was really, really deeply horrific and I think it was at a time where I felt like I couldn't really talk about it."
She added, "He was their white Prince Charming and they considered he should be with someone white and blonde."
FKA recalled how fans would compare her physical appearance to a monkey, describing, "Say if I was wearing a red dress, they would have a monkey in a red dress or I was on a bike, they would find a monkey on a bike."
"I just remember it had this massive, dysmorphic effect on me for about six months to a year, where every time I saw my pictures, I would think, 'Gosh, I look like a monkey and people are going to say that I look like a monkey, so I need to really try and hide this monkey-ness that I have,'" she continued.
FKA is now "very confident" in her appearance, but she recognized, "It was deeply unfair at the time that I was made to feel so self-conscious and so ugly. It was a lot. That period was a lot."
As for why she never spoke extensively about the bullying she faced, the "cellophane" artist said that she doesn't entirely know, but thinks it was because she's Black and came from a "low-income family" from Cheltenham, England.
Moreover, she said it happened at a time when she had to "genuinely work twice as hard at everything I do to get a seat at the table."
Now, almost three years after they ended their engagement, she has a different perspective on the experience, explaining, "I feel like if I was going through it now, I would be able to talk about it and do some good with it."
At the time she stared dating Robert, the Honey Boy actress did tweet that she was "genuinely shocked and disgusted at the amount of racism that has been infecting my account the past week."
She later said the bond she shared with Robert outweighed her desire to escape the bullying. As she put it, "The positivity that I get from [my relationship] makes the more challenging aspects...very worth it."
Additionally, Robert, who faced the same relentless abuse from Twi-hards, decried the way fans treated her in a 2017 interview with Howard Stern. The Batman actor rebuked, "They get so addicted to just wanting to cause hurt and pain on someone... It's one of the most difficult things to know how to confront. It's a faceless enemy."
Some months later, the couple quietly went their separate ways, but Robert has said that he's on good terms with all of his exes, FKA included.
The same cannot be said for FKA and her Honey Boy co-star Shia LaBeouf.
Last December, the actress accused Shia of "relentless abuse" and knowingly giving her a sexually transmitted disease.
Shia denied the allegations in a statement, saying in part, "Although many of these allegations are not true. I am not in the position to defend any of my actions."