It is not possible to have a blockmodel for a cast & uncast fishing rod at the same time. As soon as a "fishing_rod" model exists the "fishing_rod_cast" model is overwritten.

In the attached pictures:
Cast fishing rod model= gray cube
uncast fishing rod = black cube

Test resourcepack is also attached. To replicate you can activate/deactivate the two models. They both work individually, but not together.

This only seems to be a problem for blockmodels. Retexturing fishing rods still works.

Probably related to the fix of https://bugs.mojang.com/browse/MC-85101