I loaded up a world and at first all the item frames had dropped as items and then when I checked my chests, there were no items to be found.
Related issues
duplicates
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citybuilder45
Yes, I opened it afterword with 1.7.4.
Going backwards in versions is NOT supported in any way, thus that is not considered a bug. When setting the launcher to a snapshot version, it says to make backups so you need to restore to one of those.
CubeTheThird
Duplicate of MC-45009. As indicated, downgrading is not supported.
Item frames being empty is a duplicate of MC-45826
As for chests - did you open the world in a newer version, then go back to a previous one?