Check all your water parameters obviously, but if you observe this behavior long term with no other issues apparent in his health? It’s just his behavior. I have the exact *exact* same behavior in just one single male of my overstocked 36 tall bow front. He’s even the smallest of the Platys.
It’s due to the … boastful nature of the other males plus his natural shy personality. I too thought my Platy was sick, but unless he shows signs of illness? I would say be patient and observe. My shy Platy boy has been this way 6 months and he’s just shy. But healthy. He hides a lot. A LOT. But he’s playful and almost artful at escapes from anyone he doesn’t want to be around ( every fish except my honey gouramis ((trichogaster chunas))
Don’t fret too soon. I have two females, and four males! They get pushy with each other but like happy brothers. Mostly they preggo up my females and bully them for sex more than each other. They have never shown any aggression other than playful chasing yet my shy boy will have none of it. He only clamps when in the open. When he’s in his spot he’s fit as a fiddle and unclamped. I have one female isolated as we speak in week four of pregnancy waiting for fry.
Now this is in an overstocked peaceful setup of 9 harlequin rasbora, 4 dwarf gourami, 4 honey gourami, 4 black kuhli loaches, a pleco, 3 male guppies, a female guppy, 4 cardinal neons, 2 neon tetras, 4 glow light tetra, 4 male Platy, 2 female Platy (one preg in isolation), a rose tail male betta, a neon yellow glofish female betta, a cherry shrimp, 3 bumble bee shrimp, 8 ghost shrimp, two Inca snails, one nerite snail, a litter of ghost shrimp larvae, and every single resident is happy and healthy. I worked at this tank for months to get to this point and I don’t have so much as a single nipped fin, or flaked scale from any aggressions and my fish all have a spot and dance together daily. It’s truly truly a gorgeous and beautiful sight. I can watch this tank for hours doing nothing else and it took weeks for me to accept this male Platy is just a shy boy.
Sadly I raised two three spot gourami in this tank only for them to outgrow the peace of the community and have to be relocated to separate tanks. Sigh. I wished they could stay chill but I gave up having a single larger showcase fish (the blue gourami) for the well-being of my trichogaster chunas after I raised the second blue and she grew into a bully too. The three spot gourami would always mature and chase them. I favor my honeys, they are by far and away my favorite fish so I tolerate zero bullying. I have another tank that is exclusively 15 honeys in it. And another tank breeding a pair. And another tank with a betta, and another tank with a betta, and another tank with bully three spot gourami number one, and another tank with my recently relocated bully three spot two, and another tank with 5 glofish tetra.
These tanks are maybe an obsession but I will tell you, they grew out of making this peaceful overstocked masterpiece. Learning fish hierarchy is a hard lesson, sometimes the least likely suspect is a bully, sometimes the most likely is the victim. I ended up with 7 “ extra “tanks just because I wanted peace. Just observe and good luck!