Yes. it is or, at least, not uncommon.
What you have done is exposed the higher moisture content inner wood and
unequilized the tensions in the wood.
Put it aside for a couple of days and it should start to straighten out as
the newly exposed wood gives up some of it's moisture. .
Any freshly milled wood, especially if it has only been milled on one side,
should be allowed to acclimate for at least a couple of days after it has
It's always a good idea to mill a couple of fractions oversized to allow
yourself some wiggle room should the wood not return fully to the desired
shape after it has acclimated.
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