Well this is a minor detail, I think it happened when I added the new delete signals button.
https://github.com/aburch/simutrans/commit/587ecfd4d5624c95f4f71c41bff7e3c5b43137bd (https://github.com/aburch/simutrans/commit/587ecfd4d5624c95f4f71c41bff7e3c5b43137bd)
Indeed, fixed in r10492