Hello and welcome to the forum!

New ponds experience a phenomenon referred to as "wicking" - it's essentially the dry surrounding soils absorbing water in a newly hydrated pond. It can take weeks or months before the soils are saturated and the pond levels out. 1.5" seems a bit accelerated however. I recommend installing a vertical water loss measurement device [stake, pvc, etc.] and monitor daily water loss between rain events. This data is critical. I would allow the pond basin/sidewalls 2-3 months to fully saturate before being concerned with a leak.

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

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