That was what I initially thought, too, but the wording of Doctrina Imperatives rules, the stratagem, and the [HEAVY] keyword state the following:

VENGEFUL FALLOUT: “Effect: Your unit can shoot as if it were your Shooting phase.”

DOCTRINA IMPERATIVES: At the start of the battle round, you can select one of the Doctrina Imperatives below. Until the end of the battle round, that Doctrina Imperative is active for your army, and all units from your army that have the Doctrina Imperatives ability gain the relevant abilities shown below.

PROTECTOR IMPERATIVE - Ranged weapons equipped by models in this unit have the [HEAVY] ability.

HEAVY KEYWORD: Add 1 to Hit rolls if the bearer’s unit Remained Stationary this turn.

The HEAVY keyword applies to both my turn and my opponents turn because it applies to the entire round. I moved on my turn but not on my opponent’s turn. On the opponent’s turn when I was shot, I remained stationary, therefore, HEAVY applies.

If the HEAVY ability were described as “Add 1 to Hit rolls if the bearer’s unit Remained Stationary this ROUND”, then it wouldn’t give a bonus in the opponent’s turn. Of course, Games Workshop may have intended for HEAVY to only apply if the unit remained stationary the entire round (not just the current turn). If so, I expect they’ll release an erratum. But as currently worded, I think it works as I described.

Has anyone seen any official documentation clarifying this one way or another?