Catalyzes a carbon-carbon cleavage reaction; cleaves a 2-hydroxy-3-methylacyl-CoA into formyl-CoA and a 2-methyl-branched fatty aldehyde. Binds 1 Mg(2+) ion per subunit. Binds 1 thiamine pyrophosphate per subunit. Homotetramer. Belongs to the TPP enzyme family.