I would suggest the TTN antenna’s location relative to the TV antenna is going have a huge influence on its performance. It looks like the active part of the antenna is about the same height as the TV antenna and as a result the radiation pattern of the TTN antenna will no longer be uniform in all directions but distorted and directional. In comparison, the antenna to the right of the TV antenna would work correctly as it’s above the TV antenna.
To explain: if you look closely at the TV antenna you will see the cable is connected to 2 or 3 of the metal elements on the TV antenna (driven elements), the rest of the metal elements are passive. However they have the correct length and location to interact with driven elements to achieve a highly directional pattern to improve the performance of the television.
Now back to the TTN antenna: The TV antenna is going to act as Passive Elements to the TTN antenna and influence its radiation pattern. I realise the distance between the two is quite a few wavelengths and at that distance a single upright pole (like the pole that supports the TV antenna) has very little effect, however the TV antenna is large and will influence the performance of the TTN antenna.
When you conduct your “line of sight tests”, see if you can repeat this in a number of directions and see if you observe the gain is different for each direction.