X1 needs to rebuild index every day

Each day when I start X1 it needs to rebuild the entire index. I think it has to do with files in Dropbox which are missing when I open X1 but are there after it finishes Indexing Files. I have over 50,000 files in Dropbox so it's a fairly long process each day. Is there something I'm doing wrong that causes this?

I'm using X1 Search 8.2.1 build 5001hb (64-bit).

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