Moving to new address but can't select unit number

I'm a current Xfinity customer trying to transfer my Internet service to a new address a few weeks from now but can't enter the unit number at the address I'll be moving to, even though others at the same address can be selected, via the interface at https://www.xfinity.com/learn/moving. Whenever I enter the address, I just hit the message "No results found, please search again or use the form below" and entering the appropriate information in the boxes which appear followed by clicking "Keep Moving" simply pulls up a box which reads "Looks like there's multiple units in your building. Please select your apartment number." and a choice of units which aren't the one I'll be moving to.

The interface doesn't provide any explicit error, but other forum posts seem to indicate that this behavior suggests there's a block on the address due to an outstanding bill, but as an existing customer from a different address I don't see why I shouldn't be able to select it.