I was underground on my most progressed server. I went to my underground chests, and they were both empty. I went to my above ground chests and they were all empty too. My inventory still has all the items I previously had. My items weren't anywhere around the chests. I checked my other servers, and their chests were all still full. Only my one server had emptied chests. All of the items in all 8 of my chests were gone.

Update: I had been working underground for a long time. I stopped playing Minecraft for about a day. I started playing again, and went straight to mining. After all three of my pickaxes had run out, I went to the chests to gather materials, and all was gone. I checked all my chests, and they were empty. I checked chests on different servers, and they were untouched. I restarted my phone to no avail.