The exact same thing happened to me!
I had about 1700 emails in my inbox, and now I'm down to 100. I've checked the deleted items and recoverable items on office 365 web interface, and I can't find those mail in either folder. Loosing data is absolutely not acceptable
I can very clearly see emails just vanished since I have the reply I sent back, but not the original items. I restored all recoverable items into an inbox subfolder and used the search engine.
Is there any way to recover at least some of that data ?
Any inbox replication and versioning ? any kind of backup ?