I don't really think that's odd at all. His typical supporters aren't very smart, and probably have memory spans similar to that of a goldfish. They will soon forget whatever problems occurred as a result of the shutdown.
I think the bigger challenge will happen further into tax season, when more and more people find their income tax refunds are smaller than they were last year. (Note: Much of that is due to problems calculating the amount to take off on each pay check, but it should still affect Trump's popularity.)