ANTLR4 getCharPositionInLine() not working as intended in LEXER rule

ANTLR Grammer:

grammar Test;

/*
 * Parser Rules
 */
rpg_file                    : f_section
                              EOF? ;

f_section                   : f_line* ;

f_line                      : f_start f_content f_end ;

f_start                     : F_START ;
f_content                   : F_CONTENT ;
f_end                       : ANYTHING_BUT_NL* NEWLINE;

/*
 * Lexer Rules
 */
fragment F                  : ('F' | 'f') ;
fragment LOWERCASE          : [a-z] ;
fragment UPPERCASE          : [A-Z] ;
fragment DIGIT              : [0-9] ;
fragment CHAR               : (LOWERCASE | UPPERCASE) ;
fragment DIGIT_CHAR         : (DIGIT | CHAR) ;

fragment WHITESPACE         : (' ' | '\t') ;
fragment WS_OR_TEXT         : (WHITESPACE | DIGIT_CHAR) ;

fragment F_FILENAME         : WS_OR_TEXT?
                              WS_OR_TEXT?
                              WS_OR_TEXT?
                              WS_OR_TEXT?
                              WS_OR_TEXT?
                              WS_OR_TEXT?
                              WS_OR_TEXT? ;

/*
 * First 5 characters in RPG format are unused and should be ignored
 */
fragment SPEC_START        : {getCharPositionInLine() == 0}?
                             ANYTHING_BUT_NL
                             ANYTHING_BUT_NL
                             ANYTHING_BUT_NL
                             ANYTHING_BUT_NL
                             ANYTHING_BUT_NL ;

F_START                     : SPEC_START F ;
F_CONTENT                   : {getCharPositionInLine() == 6}? CHAR F_FILENAME ;

ANYTHING_BUT_NL             : ~('\n' | '\r') ;
NEWLINE                     : ('\r'? '\n' | '\r') ;

ANTLR Input:

     FFILENAMEIF  E           K        DISK
     FFILENA01UF  E           K        DISK

In the RPG files that I am trying to parse, there are specifications that are found by looking at the character in column 6 of a line. If there is an 'F' in column 6, it means its a file description specification and the following 8 characters are the file name. I am trying to extract the filename in the f_content parser rule using the F_CONTENT lexer rule.

The expected output here is that the F_CONTENT lexer rule only matches when the sequence starts at position 6 (column 7) of the RPG file, thus extracting columns 7 - 14 which are reserved for filename. The actual output is that the filename is extracted, but then for the next 8 characters afterwards, it also matches to F_CONTENT, meaning the getCharPositionInLine() is not being applied as intended.

I also have getCharPositionInLine() in the SPEC_START lexer rule, but that isn't working either in other cases when full code is provided (it is matching to sequences that are in middle of line 30 characters in).