If you select a placement location and then look at an area you cannot place a portal on, the preview of the portal location gets stuck at the last position where it was possible to place it.

You can still confirm the placement of the portal if a red line / unplaceable is displayed in the preview, which allows you to place a portal miles away.

Potential fix: Do not allow the portal to be placed if the preview shows that it cannot be placed, or enforce a maximum distance