I thought about what you said before you posted and perhaps the shield was not activated.

But I am in the same camp that I think the probe travelled a long way before reaching the planet.

Here is the excerpt from the script weather you count it as canon or not

EXT. GALAXY - PLANET HOTH

A Star Destroyer moves through space, releasing Imperial probe robots
from its underside.

One of these probes zooms toward the planet Hoth and lands on its ice-
covered surface. An explosion marks the point of
impact.

The 4th draft by Kasdan which is the "shooting script" does not have this excerpt it starts with Luke on Snow lizard or Taun Taun.

The fith draft is unavailable but was an edit of the fourth draft from page 136 on wards which finalized just before shooting started. there eere a 3 further subsequent revisions to the 5th draft whilst during shooting. seeing as the opening with the star destroyer is all special effects based. I also going to say this sort of thing has Mr Lucus stamp all over it rather than Kasdan