The SaveFSR and RestFSR macros along with INT_CREATE_H, INT_CREATE_L, and INT_RETURN all use MOVFF to save the context.
MOVFF can only address the first 4K of ram and the K42 is part of the new 18XV core devices that can address up to 16K of ram.

In these new devices the SFR registers have been moved into the upper banks past where the MOVFF instruction can address.
The new MOVFFL instruction has to be used in its place to address the full 14-bit ram space.

I haven't used one of these yet but the whole context saving mechanism of dt-ints may not be needed with these.
The new core includes a two-level deep context stack which saves the STATUS, WREG, BSR, FSR0/1/2, PRODL/PRODH and
PCLATH/U registers automatically for you into the shadow register memory located in bank 56.