LUIS: intent utterances disappear randomly

Question

Hi.

First to describe the issue I'm having right now,
I made several LUIS apps for intent classification and then created intents, uploaded utterance examples.

But when I tried to train one app, it failed with the message 'Some intents have no utterances or patterns.'

It makes no sense because every intent has at least 1 or more utterance examples.

Found out that for some intents (randomly selected but fixed, I guess), utterance examples disappear right after the data upload.
And some of them lose all utterances (for example, one intent lost all total 34 utterances), making train not possible.

This 'disappearing-data' issue occurs at all other apps, too.

1. I want to know why this issue occurs
2. and how to solve this issue in order not to lose any upload data.

Thank you.