"When HIV is no longer detected in someone’s blood test, they cannot transmit the virus to sexual partners."
I do not believe this to be true.
I think the virus is transmitted, but in such low levels that it takes much longer for the infected to register a positive test. And by that time they do not suspect that the actual source was the source.
Here's a study on the concern of people on PrEP donating blood and how they can be HIV+, but the test equipment can't detect it.
https://www.ncbi.nlm.nih.gov/pubmed/28370177
If it's a bad thing for HIV- people to receive blood from someone who is HIV+ (on meds, undetectable) then OBVIOUSLY it's also bad for a HIV- person to receive the semen of a HIV+ (on meds, undetectable) person.