In their paper at the International Symposium on Multiple-Valued Logic in 2017, C. Moraga, R. S. Stanković, M. Stanković and S. Stojković presented a conjecture for the number of fixed points (i.e., eigenvectors with eigenvalue 1) of the Reed-Muller-Fourier transform of functions of several variables in multiple-valued logic. We will prove this conjecture, and we will generalize it in two directions: we will deal with other transforms as well (such as the discrete Pascal transform and more general triangular self-inverse transforms), and we will also consider eigenvectors corresponding to other eigenvalues.