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The behavior for party members that are just slightly too far away is pretty strange. When selecting Cure or other actions and targeting players that are a bit far off in the distance, they are targeted for a moment, then the target is forcibly removed, and the cursor returns to the action bar.
When the distance in which you are able to target them is exceeded, the cursor is set to return to the action bar. It can be thought that you are right on the border of being able to target/not being able to target, so try bridging the gap just slightly.