This is possibly what's going on:
- Any item placed outside the little sectioned-off rectangle around Flamey can possibly disappear. That's just how the game works since the island frequently regenerates in slightly different ways.
- If a decorative item disappears in this manner, it should reappear in your mail eventually--talk to Postal Bear to check your mail. If it doesn't reappear in your mail within a day, that's a bug.
- This only applies to real decorative items, not to things like food and shells. If you leave food and shells lying around, the game will clean them up eventually even if you've put them in the sectioned-off area.
- You'll eventually discover some other sectioned-off parts of the island. Decorative items placed there are similarly protected from disappearing.
- You certainly can (and in some cases, should) place certain items in the wilderness but just be aware that you may occasionally have to go get it back from Postal Bear if it disappears.
- The areas around bears are especially likely to make items disappear.