https://reference.opcfoundation.org/Core/Part6/v105/docs/5.1.13

This section says:

The following rules are used to convert a DataType Name or a Structure Field Name to a string supported by the Encoding:

1. Any character that is not permitted by the DataEncoding is replaced by an underscore (U+005F);
2. A name with text sequence that is not valid in the first position has an underscore (U+005F) added as a prefix.

But the examples it lists in Table 8 do not consistently apply these rules.
_Hello should become __Hello