Hi MostlyRum, welcome. Yes this is the perfect place to ask.
Is this happening on all the Cleo missions or just the Valley campaign? Once the tombs in the Valley missions are finished and all goods dispatched they appear to disappear into the cliffs. On the mini-map you can still see the dark grey over the cliffs where it was placed.
If you go to the overlays, or press H, you can hide the cliffs. In addition to helping you place tombs on their limited specific spots, you should be able to see the old completed tombs.
Otherwise, if this isn't your problem, I'm not sure what can be the cause, this is a new one to me. But to narrow down possible issues, can you give more information
- Which version of the game are you playing?
- Is it only Valley missions or do the monuments not carry over in the Alexandria missions as well?
- Do any of the Cleo missions allow you to carry over your most experienced soldier fort
Eagles may soar, but weasels do not get sucked into jet engines
Two Eskimos sitting in a kayak were chilly, so they lit a fire in the craft. Unsurprisingly it sank, Proving once again that you can't have your kayak and heat it too