What's actually happening
Your sliding door isn't supposed to be perfectly dry inside the track. It's designed to collect a little rainwater in the outer channel and drain it back outside through small slots called weep holes.
When those slots clog, water backs up. It rises in the track until it clears the inner lip, then it spills toward the inside of your house.
Downstairs, that spill just wets the floor and you mop it up. Upstairs, the same water can slip past the track edge into the floor framing, then travel sideways to the lowest point before it shows up as a ceiling stain in the room below.
That's the frustrating part about second-floor door leaks. The evidence often appears several feet away from the actual leak, so people chase the wrong spot for weeks.
The usual suspects, in order
in most cases this comes down to drainage or sealing, not a broken door.
Clogged weep holes lead the list. They're the most common cause and the easiest to fix. A zip tie and five minutes usually tells you if that's the issue.
Next is the sill pan, the waterproof tray under the door that's supposed to catch overflow and send it back out. On a lot of upstairs doors it was skipped during construction or installed backward. When that's missing, any water past the track goes straight into the floor.
Weatherstripping is third. The seals flatten and crack after a decade or so, and once they do, wind-driven rain blows right over the track lip during storms.
Then there's flashing and caulk failure above or beside the door. Water gets in up high, runs down inside the wall, and pools in the track looking exactly like a track leak. Upstairs doors get hammered by more wind-driven rain, so this is more common than most homeowners expect.
Settling and roller problems round out the list. They're less common but worth checking if everything else looks fine.