The original (Exuberant-ctags) parser handles #define C preprocessor directive and C style comments by itself. In Universal-ctags Asm parser utilizes CPreProcessor meta parser for handling them. So a language object defined with #define is tagged as "defines" of CPreProcessor language, not Asm language.

$ cat input.S
#define S 1

$ e-ctags --fields=+l  -o - input.S
S    input.S /^#define S 1$/;"       d       language:Asm

$ u-ctags --fields=+l  -o - input.S
S    input.S /^#define S /;" d       language:CPreProcessor  file: