to throw a spanner in the works it is show based.
for example Boston Legal seems to find subs (opensubtitles.org) and Rugrats don’t want nothing to do with it until i rename the file.
edit
the more shows I look at the more this seems to be confined to more fringe cases, I checked Top Gear and Superior Donuts, both were happy to find subs. Timeless finds nothing, rename file can now be found.
further edit
The “random” subs that are found are the same sXX and eXX just the showname is not found.