How do I stop this from happening?

[Der Kuenstler]

Tactical note- the test shows you want enemy tanks to approach your inf buttoned so they'll hurl grenades at it rather than spray it, so it might be a good idea to order a squad or MG/ light mortar etc (behind the front line somewhere) to fire at tanks to force them to button before they get close to your front line grenade-throwers.

Good advice - unless the tank happens to roll up already buttoned and then suddenly unbuttons before the guys can throw grenades like in my case. :(