Topic on User talk:Les
From G54 to GvKG, Ifukube was credited under 音楽 ("music") but beginning with GvMothra he was given the title 音楽監督. Though 監督 would usually be translated as "director", I went with music supervisor because of its use on those movies' English-language Japanese VHS covers (1 | 2 | 3). Also, though Monster Planet doesn't have credits, the book Heisei Godzilla Perfection gives them, and he's music director/supervisor there too.
We can only speculate on what changed between GvKG and GvMothra, but it's definitely not meant to imply that he didn't compose for the movies. If anything, I think it implies a higher level of control over the music and not less. Although realistically, it was probably just a nominal change out of respect for his work. I hope that clears it up!