I can't confirm this with data but my guess would be that even though unemployment is low, people are generally doing more work for less money - and everything costs more (especially housing)...but that's just what income inequality looks like.
Sure: Employment and "has enough money to live" are not the same thing. The second one is a far more interesting metric, but also far more sober for most governments, so they don't use it.
I find it hard to imagine that so many people have jobs and live on the street at the same time. “Working homeless” is a thing but typically means couch surfing or car camping, not rough sleeping.
There hasn't been a massive explosion in homelessness in San Francisco. SF's homeless rate hasn't changed much in 15-20 years, it was just as high during the last economic boom in 2005-2006. It was nearly as high on a population ratio basis during the peak of the economy in 2000 as well, despite a lower U6 unemployment rate at that time.
General homelessness was 5,376 in 2000. It was 6,248 in 2005. It was 6,686 in 2015.
SF is doing an extraordinarily poor job of managing its homelessness, despite their affluence and vast resources. That's the fundamental problem.
Homelessness in SF isn’t actually exploding, it’s getting more visible because development (to accommodate the labor force boom!) is steadily chipping away at the “out of sight, out of mind” areas where tent cities used to hide. Such sites receive most/all development because the NIMBYism and anti-gentrification activism are so much stronger in populated areas.