Tested on Neoforge, when using Forgified Fabric API Tectonic will crash the game when you go to single player menu if you have no worlds or press create new world when you have some. Crash report: https://mclo.gs/TcDi1cc
When using these mods by themselves separately, everything works, the crash only happens when both of them are present.

Using the fabric version seems to fix this crash for me

It works perfectly fine on fabric, well it works on neoforge fine on its own too as it does not need any additional mods or anything, but having ffapi with tectonic installed crashes the game. Connector and thus FFAPI plays an important part when gathering mods and in this state Tectonic pretty much kills it completely as you can't use both of them.
Tectonic is also the only one of these kind of mods that I know that causes the crash, for example Terralith or Lithosphere work with no issues at all, unless I don't understand something ofc.

Not quite sure what the issue is, but if you use the fabric Tectonic version it works fine. Likely an issue on FFAPI's end.