This is a known, rare and (almost always) random and unreproducible issue. Loading the earlier save and triggering the act transition again should get the proper storage chest placed on stage. This has been reproducible in one reported case, when it was triggered by a specific mod; disabling the mod avoided the problem when redoing the act transition.
If you are playing on PC, please email firstname.lastname@example.org with both saves (or email/post/PM a Dropbox or Google drive link, etc).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
In Windows Explorer you can zip a file or folder by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder' (and similarly in OSX/Finder).
PS4 saves can be sent, as well, but they would need to be transferred to a USB flash drive first.
In the Saved Data in System Storage section of the Settings menu, click Copy to USB Storage Device, select the relevant save(s) and Copy. From there you can zip the files, then copy to email or upload somewhere and provide a link.