The substrate I used was a key to success I think, cause it sticked together quite well like a clay and larva seemed to had easier time compacting the soil around it. What was really interesting is that its gut was pretty much empty before it built third cocoon but it managed to hold the substrate together to build a cocoon by making substrate damp (I believe it used its body fluid for this). Honestly, I thought it wouldn't make it when I broke 3rd cocoon cause by that time, larva had shrunken to about 2/3 of its original size but it still made it through
It would be great to pull out the 4 broken cocoons and take photographs to document such an amazing occurence. Since they are flower beetles the cocoons have a double wall and the peices will still be around to photograph unless you threw out the substrate.
I cannot know what you saw but I am rather familiar with this species. I have kept Osmoderma eremicola for a few decades and have never seen an animal make an actual pupal cell more than once. It may not be impossible but it is highly improbable. However, something most people do not know about this species is the L2 and L3 make overwintering cells (that are not hardened or constructed using anal secretions) which they can remake over and over again. Most species make molting cells and pupal cells but this species also makes a unique overwintering cell. The attached photo illustrates this odd behavior. There is a mature L3 and a mature L2 each in a cell and a broken cell from another L3 showing the construction style. They only make this third type of cell in the winter but they will make it as warm as 74F. This cell is usually made against the bottom of the container so that a "hole" exists and the animals can be seen (in small clear containers). This hole is visible in the break away cell. Pupal cells rarely, if ever, are made with a hole and there is an outer and inner surface to the pupal cell.