You've got at least two things working against you in lower light.
- Less contrast for your camera to use to focus
- Less usable signal (light) with which to make a picture
Since both phase detection autofocus (SLR using the viewfinder) and contrast detection autofocus (Live view or mirrorless) uses contrast to determine what is in focus, less contrast in the scene gives the camera's AF system less to work with. Most cameras are programmed to focus as accurately as possible without taking too long to do it. As a result, in low light such cameras will be less accurate with regard to AF.
A lower signal to noise ratio usually means the camera will apply more noise reduction in dim light, even at ISO 100, than it will in brighter light. Noise reduction tends to reduce image detail along with the noise.
Focusing, both auto and manual, are usually done when the aperture is wide open. The camera will only stop down to the selected aperture value the instant before the image is captured. The wider the aperture of the lens, the better the camera's autofocus can perform. Your 18-270mm f/3.5-6.3 lens is relatively slow, especially on the telephoto end where wide open the aperture is only f/6.3. That's right on the fringe of what most cameras' PDAF systems are capable of using to AF at all. Even when shooting at narrower apertures, a wide aperture lens gives an advantage to AF systems over a narrower aperture one. You might try using Live View based contrast detection AF instead. It will probably be much slower in low light, but should be more accurate if you've managed to properly tell the camera where it is you wish it to focus. Your sample image, for example, is very well focused on the area with the highest amount of contrast in the scene at the extreme lower right.
