Suggested resolution:

Add a new paragraph 4 at the end of 7.6.1.2 [expr.sub]:

If the subscript operator invokes an operator function, the sequencing restrictions of the corresponding function call expression apply (12.4.5 [over.sub], 7.6.1.3 [expr.call]).

The specification about the relative sequencing of multiple parameters of the subscripting operator is missing. Also, issue 2507 adds support for default arguments for user-defined subscripting operators, but the sequencing of these is unspecified, too.