The %T pattern can be used in file names to add the title of the window being captured. In case of fullscreen capture when there is no title spectacles removes this pattern, but also surrounding separators.
Unfortunately it is too agressive in removing separators and will also eat the % character of the closest following pattern in some cases. This breaks patterns like %T_%Y-%M-%D_%H%m%S.
See https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=953536