Vaal Temple forge eating weapon support gems

I had a voltaic staff with 3 support gems (3 slots) that I put into the forge to upgrade the quality directly from my equipped inventory.

When the completed item dropped to the ground only one of the support gems appeared with it. The other two went missing. I note that it was the gem in the first slot that appeared.

This wasn't an issue with the same type of staff that had only 2 support gem slots that were filled.