One more thing to check.
Did you correctly transfer the $7,500 credit to Schedule 3 line 54 and 55 and did you enter 8936 at line 54b and check the box?
Did line 55 get correctly entered on line 12 on the 1040 again along with a check box on line 12b?
If you did all that correctly, and line 11 from your 1040 is >or= the $7,500 (minus any personal credits from the 1040, see instructions) then you got full use of the credit. That doesn’t necessarily mean you will get it all back in a refund. Other items can reduce it such as Other Taxes on line 12 (from Schedule 4) or tax penalties or unpaid taxes or corrections from previous returns to name a few.
But if you did everything correct, you benefited from the full $7,500 by reducing your tax liability by the $7,500 credit, no matter what your actual refund was.
And remember, any Federal income tax withheld or tax payments made by you for 2018 have no bearing on how much of the non-refundable non-rollover credit you get to use. Only your tax liability calculated from your taxable income on line 10 of the 1040 and then placed on line 11 determines how much of the $7,500 credit you can use.
If it’s >or=$7,500, then you get to use it all; if <$7,500, then you can only use the lesser number.
No doubt the forth coming IRS letter will explain why your anticipated refund was reduced. As others have mentioned and based on the limited information you have shared, I think that you got to use the full $7,500 credit and something was flagged by the IRS computer that then reduced your refund.
Please let us know what you find out.