As of vanilla 1.14, signs must be made using identical wood, instead of mixing and matching, and the output sign matches the wood type used to craft it. This feature could be implemented in 1.12.2 TerraFirmaCraft with its wood types.

Been noted in https://github.com/TerraFirmaCraft/TerraFirmaCraft/projects/6#card-29397197 instead at it's not a current issue.

We aren't trying to backport 1.14 features to 1.12 TFC - If vanilla does a thing in 1.14 we will add similar features when we move to 1.14, i.e. sign variants, stripped logs, log vs wood blocks etc.