Who is telling this story?
As it turned out the stranger was Roberto
First you tell us this is "a stranger", then the very next sentence it is someone known as Roberto. From which POV is he first "a stranger" then "Roberto"? Why has the POV suddenly changed?
If the POV is following Isabella through limited 3rd-person, then the man on the beach is not a stranger, she just isn't sure who it is is at first. "Stranger" seems like the wrong word for Isabella to use.
Then, within the same paragraph the POV flips. An unknown "someone" approaches Roberto and he pulls himself up to see it is Isabella. Suddenly we are within Roberto's thoughts and perceptions, and Isabella is the "stranger".
The confusing part of this narrative voice is not how it jumps around but that it keeps resetting to ignorant. This is not an omniscient narrator, it's an amnesiac narrator that is describing visible actions and blocking (how people move around the stage), but it doesn't have a consistent anchor POV to follow. By the end of one paragraph the person introduced as "a stranger" is sharing his most intimate thoughts.
Who is the main character we are meant to relate to? Isabella or Roberto? Every time we establish a POV, it get's broken in the next sentence. There is no continuity.
Try the narrative voice called limited 3rd-person. This voice closely follows the POV of one character in the scene, but told in 3rd-person. The "limited" POV means we can hear one character's thoughts, but the actions and reactions of other characters are only observed and described through the anchor character – usually this is the person who has stakes in the scene: our MC.
Try re-writing the scene as limited to one anchor character only. Assume the narrator knows everything they know, but only what they know. This orients the reader to which POV we are meant to view the situation. We automatically sympathize with the POV character, they are our emotional and perceptual anchor.
Isabella as anchor POV:
A body was lying, face into the sands, belly down, on the beach. Isabella kept her head and ran to investigate, but as she approached, the body stirred, pulled itself up onto its hips and turned to look her.
It was just Roberto. He stared at her, the short skirt and crop top she was wearing, and grinned like a kid on Christmas who had received a puppy under the tree. Isabella looked around uncomfortably. He was the last person she wanted to see right now.
Roberto as anchor POV
Roberto found a spot on the beach away from the sports goons and noisy
children. He needed a plan for tonight, some way to get Isabella
alone. Tonight was his last chance to tell her.
The sand was cool, but the sun was already hot. Roberto sat thinking,
but no idea would come. That bright sun beat down, and only made it
harder to concentrate. Isabella was popular, she was never alone.
Sweat stung his eyes. The sunblock lotion and his sunglasses were back
at the noisy volleyball game. Roberto rolled over onto his stomach and
hid his face in the sand. Their happy screams and laughter only made
it harder to think.
He wasn't sure how long he'd been lying there, waiting for an idea to
strike, when he heard footstep running towards him then stop short with
a little gasp. He sat up. It was her! Isabella, alone! She had on a
crop top and short skirt – she looked wonderful. Roberto knew he was grinning like a fool.
Isabella frowned. "I thought you were a dead body!" She sounded
How do we properly manage the state of transition between unknown and known?
Limited 3rd-person knows what the character knows, and that's it. This narrative voice has no responsibility to inform the reader of anything, it's just a reflection of the thoughts and opinions of the MC. As far as the narrative voice is concerned there is no "reader" to talk to.
The reader infers what is known and unknown by the character, through what they do and think. The reader may actually understand more than the character, through observed clues and actions the character misinterprets, or because other scenes follow a different anchor character so the reader has heard and understood their thoughts while they held the stakes for the scene.
You can also jump around to various POV within a scene, but keep the POV switches to different paragraphs the same way you would separate the dialog into paragraphs every time the speaker changes.