It's a common shock: you hide a photo on your phone, and it appears on the iPad everyone in the house uses.
Why it happened
The iPad is signed in to your Apple account with iCloud Photos turned on. Hidden photos sync too (details).
Fix it today
- On the iPad: Settings → Photos → Use Face ID (or Passcode) so the Hidden album is locked.
- Better: sign the shared iPad into a separate Apple account, or turn off iCloud Photos on it.
- Set up Family Sharing so family members have their own accounts and can still share purchases.
Long term
Keep truly private photos in an encrypted vault on your own phone and nowhere else. Leave device sync off for vaults you don't want on other devices.
Try Veilbox free
Encrypted on your iPhone. No account required.