water in toilet bowl sometimes disappears

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I have a one-piece toilet. After I flush, the bowl fills up as expected, but about 75% of the time, most of the water in the bowl will have disappeared within 30 minutes. This means I have to flush twice the next time.

If the water disappeared all the time, I'd assume there was a crack in the bowl, but as I mentioned, the water doesn't always disappear. Does anyone have any idea what's going on?