Grompp fails with Atomtype starting with number and ending with underscore

GROMACS version: 2021.2 and 2019.4
GROMACS modification: both patched with plumed (2019.4) and non modified (2021.2)

I noticed that if there are some Atomtypes that start with a number and end with an underscore grompp will fail with this message:

ERROR 1 [file, line 411]:
  Atomtype 3C_ not found

If the Atomtype only starts with a number or only ends with an underscore everything works fine

This topology files have no #include statements and all the atomtypes (even the ones not working) are defined

I cannot attach some files to reproduce the problem because I am a new user

ps I hope this is the right form where to post this kind of things