This is not a regression and has been the behavior since Go1.4.2, possibly way earlier.

The problem:

• The logic for the Writer outputs the magic bytes for the GNU format if the usedBinary flag is true.
• This flag is set to true if a numeric field could not be encoded in base-8 and had to be promoted to base-256.
• This flag is never reset, thus permanently forcing the Writer to use the GNU format even if PAX headers are used.

It is highly likely that we have been generating invalid PAX files under certain circumstances. The reason why this has not been reported yet is because the the GNU and BSD tar tools both liberally accept these invalid files without complaint. In fact, our own Reader liberally accepts this invalid hybrid of both GNU and PAX.