This is by design. When you have DBNull for one of the fields, the end result becomes undetermined and that is why we consider the whole thing as DBNull. To avoid this, you can use the IsNull built-in function e.g.:
=IsNull(Fields.MemberTitle1, "") + " " + IsNull(Fields.MemberTitle2, "") + " " + IsNull(Fields.MemberTitle3, "")
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