Apa, 22, and Robertson, 29, were seen holding hands and hugging, and — according to a source — "leaning in and kissing each other" at the bash. They were in a group that included Lili Reinhart, Camila Mendes, Cole Sprouse and Madelaine Petsch.
Along with working on "A Dog's Purpose" together in 2017, the duo will be shooting "I Still Believe" with Shania Twain.
Robertson previously dated Graham Rogers and "Teen Wolf" actor Dylan O'Brien, but Apa hasn't been publicly linked with anyone since achieving fame on "Riverdale."
In 2017, KJ told Cosmopolitan, "I think I'm just waiting for the right time, right girl. I'm pretty busy at the moment. I don't know how much I have to offer to someone right now.”
A year later, he explained why he wasn't feeling the pressure to be in a relationship, telling Seventeen, "The thing is, I don't think it matters whether you're looking for a relationship or not — it's not for us to decide when we meet the love of our lives."