The golden section search in LibV1:time_constant is hard-coded to search for a value of x (in A exp(b t) + x) between 0 and 1 (See here). But in some traces (e.g. sagtrace1), the actual value of x is more than 1 (2.7 for sagtrace1).
I intend to increase the range of search for x in my next PR.