In a pre-taped package aired before his performance, Muniz opened up about his many health problems and how they may have led to problems with his long-term memory.
"I'm not actually sure how my memory loss started," Muniz shared. "I have had nine concussions and I've had a fair amount of mini-strokes."
"I'm not saying those things correlate exactly to the reason why my memory's not great," the 31-year-old actor explained while rehearsing with his pro partner, Witney Carson.
Muniz also admitted that he's never gotten a professional opinion on his memory struggles. "I've never been to the doctor and been like, you know, 'Why don't I have a memory?'" he said. "To be honest, I've never really talked about it."
To help him remember, the actor-turned-race car driver said that his girlfriend, Paige, writes down everything they do each day together in a journal.
"I love to be able to look back," he said of the journal. "It makes make a little sad that it doesn't just pop back into my mind. Like, I should have remembered going to Australia. That's something that people remember."
Muniz's Malcolm in the Middle co-star Bryan Cranston also sat down for the package to express his love and support for his friend, and shared some sweet sentiments about his memory troubles.
"I told him not to worry about what you remember and what you don't remember. They're still your experiences," Cranston said. "That will be my job. I will tell him, 'Remember this? Remember that from Malcolm? What a life for you!'"
"My most memorable year is 2017, because I learned to live in the present," Muniz explained. "I'm happy with my acting career, I'm happy the decision I made to drive race cars and to focus on music. Even if I don't remember it all, I'm happy."
Taking to the DWTS stage, Muniz and Carson performed a fun, energetic quickstep set to Coldplay's "Adventure of a Lifetime" and the sweet dance won over the judges, who praised the star for his spirit and determination.
For their efforts, the pair earned a score of 24 out of 30, which is an improvement over their marks last week.
After getting some feedback from the judges, host Tom Bergeron asked Muniz about his health issues and memory battle. "You seem to be almost at peace with the fact that you're having memory issues," Bergeron said. "Is that accurate or is that just acting?"
"No, 100 percent. I mean, it's not something that I necessarily think of. I'm just me, so I only know my brain and my life. But it's weird when I talk to family or friends or people who I've worked with and they bring up things that I don't quite grasp unless I see pictures and videos and stuff."
Earlier this season, Muniz opened up to ET about his health struggles, including his physical therapy to help ease pain stemming from a car crash that broke his back several years ago.
"I want to make sure we are ready, so there is a fine line of, how hard do you work to how hard do you take care of your back," he explained. "We rehearse five hours, but then I spend about seven hours literally trying to heal, doing whatever I can."
Check out the video below to hear more from the actor about the variety of treatments he undergoes to mitigate his back pain in order to compete on the show.
Dancing With the Stars airs Mondays at 8 p.m. ET/PT on ABC.