Performing long lasting actions like healing oneself or a team mate transforms the player into a sitting duck. Often you can only watch in awe and count the seconds while the enemy moves in, takes aim and ...
I remember that in a previous version of the alpha there was a new action option to explicitly abort healing, but this seems to have disappeared in the meanwhile.
I suggest to automatically abort any long lasting actions like healing yourself or another character as soon as the player presses a movement key.
Less preferred alternatives might be to reintroduce the action menu entry or a specific bindable key.