Subclause 15.13 [cpp.pragma.op] paragraph 1 specifies:

A unary operator expression of the form:

  _Pragma ( string-literal )

is processed as follows: The string-literal is destringized by deleting the L prefix, if present, deleting the leading and trailing double-quotes, replacing each escape sequence \" by a double-quote, and replacing each escape sequence \\ by a single backslash. The resulting sequence of characters is processed through translation phase 3 to produce preprocessing tokens that are executed as if they were the pp-tokens in a pragma directive. The original four preprocessing tokens in the unary operator expression are removed.

The destringizing does not, but should, consider encoding-prefixes other than L.

See also C23 6.10.11.

Possible resolution:

Change in 15.13 [cpp.pragma.op] paragraph 1 as follows:

A unary operator expression of the form: form

  _Pragma ( string-literal )

is processed as follows: The string-literal is destringized by deleting the L prefix, if present any encoding-prefix, deleting the leading and trailing double-quotes, replacing each escape sequence \" by a double-quote, and replacing each escape sequence \\ by a single backslash. The resulting sequence of characters is processed through translation phase 3 to produce preprocessing tokens that are executed as if they were the pp-tokens in a pragma directive. The original four preprocessing tokens in the unary operator expression are removed.