That is expected behaviour for decades already: RFC 5322 § 2.1.1 (current form of RFT 2822 from 2001) clearly tells you plaintext e-mail lines have a limit, and phpBB enforces it by breaking after 75 or more octals. Not following this standard risks that one of the MTAs that route your e-mail won't process it (and might perform that linebreak themselves). I'm pretty sure the line length limit stems from the 1970s.
How can you call the software "poor" just because it complies with industry standards - if it didn't comply then your assertion would be correct. Maybe the standards are wrong and need updating - but that is something for discussing in another place.
It applies to every text payload. But looking from another angle at the situation you'll notice that HTML is immune to whitespaces (such as linebreaks). Again this is basic knowledge, ripe of 28 years.