The Unicode® Standard Version 15.0 – Core Specification
https://www.unicode.org/versions/Unicode15.0.0/ch03.pdf

When converting between different encoding schemes, extreme care must be taken in handling any initial byte order marks.
For example, if one converted a UTF-16 byte serialization with an initial byte order mark to a UTF-8 byte serialization, thereby converting the byte order mark to <EF BB BF> in the UTF-8 form, the <EF BB BF> would now be ambiguous as to its status as a byte order mark (from its source) or as an initial zero width no break space.
If the UTF-8 byte serialization were then converted to UTF-16BE and the initial <EF BB BF> were converted to <FE FF>, the interpretation of the U+FEFF character would have been modified by the conversion.
This would be nonconformant behavior according to conformance clause C7, because the change between byte serializations would have resulted in modification of the interpretation of the text.
This is one reason why the use of the initial byte sequence <EF BB BF> as a signature on UTF-8 byte sequences is not recommended by the Unicode Standard.