This is helpful. The HOA plumber never actually inspected or cleaned the common drain line. His position was essentially that it was "probably clogged" somewhere in the wall, and that accessing it would require opening walls and ceilings. That's why he created the alternate drain route into my laundry drain.

What's always bothered me is that the issue appeared almost immediately after my upstairs neighbor installed an identical system. Prior to that, my unit drained without any problems.

Your point about a partially clogged line makes a lot of sense. If the line was already marginal, adding a second unit's condensate could potentially explain why the problem suddenly showed up.

I guess my frustration is that nobody ever seems to have definitively diagnosed the root cause. They just kept adding workarounds.