Happened - so far - with two objects:

- Vortex Cube: if present on a planet, it is found at the entrance of caves/inside the caves.
Had to back track several caves (blind end), and I found that the vortex cubes that I had "harvested" on my way in had been back again >> BUT only until I got maybe two steps in front of them. After this (you getting nearer) they disappeared into thin air.

-Albumen pearl: same as the vortex cube, in and around caves.
These items, too, are respawning: when you first find a pearl, you have to open the plant, then take the pearl. Right after this the plant is open, showing no pearl, obviously.
If you back track, you will find the same plant, and if you open it (had been open already!), you will find one pearl again.
But, this time you can not take the pearl.

Both items are listed as extremely rare, and they sell for quite a bit of units.
Looking for these items has some kind of a price: so far each planet containing them had been very harsh!

EDIT:

This respawn seems to be a wide spread deseae among "extreme rare" items: found some red gravitino balls (additional to the green variant) with the same behavior.

But, you can use this possible bug as useful feature: backtracking is easy when you find one or more of the respawners >> you have been there already!!