Should I be concerned if there is a small gap in the mortar when tiling?
The way I read your question is that due to some misalignment in the tiles, they were removed and some of the mortar was removed and the tiles replaced with a gap underneath. If the gap was filled when the tiles were pushed horizontally, you'll be fine. If a sizable gap remained, you could have a problem with the tile cracking if you step on it just right, heel to tile. Once the mortar has dried knock on the tile and listen for a hollow sound, which would indicate an air gap. Make sure you keep a number of extra tiles incase you do develop a problem.