First, a reboot will never "revert" your completed IAP, and never should it (unless serious problems with hardware that causes data loss).
When you uninstall and re-install an app (or simply clears its data), then:
- If that app supports cloud backup, your data will be recovered and nothing is lost or reverted
- If that app has no backup then:
See Consumable and non-consumable purchases.
- Consumable purchases are like golds, gems and timed boosts that does not last forever.
- Non-consumable purchases are like premium unlocks, special characters and coin doublers that lasts forever after purchase.