MySQL 8.0 Release Notes
MySQL 8.0 Source Code Documentation
If a stored procedure exits with an unhandled exception,
modified values of OUT and
INOUT parameters are not propagated back to
the caller.
If an exception is handled by a CONTINUE or
EXIT handler that contains a
RESIGNAL statement, execution of
RESIGNAL pops the Diagnostics
Area stack, thus signalling the exception (that is, the
information that existed before entry into the handler). If the
exception is an error, the values of OUT and
INOUT parameters are not propagated back to
the caller.