Is the intended design supposed to be that the 2-corner piece will always have straights on either side?
Edit:
I’ve got it somewhat sort of but still not really quite working. I think there’s an underlying issue with how this is currently set up preventing this. Due to it using the NextSpawnPoint variable to spawn the next tile, this mean it can ONLY extend in either left or right direction.
The system is currently set up in such a way that it will continue along one single path. Due to how I changed things a little bit, I think this is possible to workaround*(although it’d be an ugly, lazy workaround as opposed to re-designing the system a bit from scratch)*.
Edit 2:
Another reason as to why it wouldn’t work is because the variable FloorCurves inside your game mode has 3 entries, but only the first index has a set value - the other 2 are set as none.
Edit 3:
Sorry for the delay, finally found some time to look at it a bit more thoroughly. It took me some head scratching to figure out how it was all set up*(no offense, but it’s pretty messy but I understand that you’re merely in a testing phase so fully understandable)*.
I made some adjustments to BPFLOORTILE, BPFLOORTILE_BothCorner and rungamemode, it’s not an optimal solution but it shows one way of how to deal with the issue. This method causes a minor lag spike due to it extending every T-section both ways to prevent it from looking half finished in the distance.
This can very easily be optimized, considering you have a lot of static meshes, I’d recommend you look into using Instanced Static Meshes, rather than every actor having several static meshes.
One way to achieve this would be to have one world-wide actor that “hosts” the instanced static meshes. And then when a tile is destroyed, make sure the instanced meshes belonging to that one are removed from the instance. This should greatly increase performance. 
You can download the updated project here. I could probably only upload the .uassets but dunno if UE4 would take too kindly to that because of all references and potential changes you might have made*(backup your original project first in case you intend to overwrite!)*. Also if you wish to have this download link removed after you’ve downloaded it, either let me know here or throw me a PM and I’ll remove it!