Quality control by the contractor is soooo important. Formicary corrosion like you described can happen if a contractor doesn't pull a proper vacuum on the system to evacuate moisture before releasing refrigerant. I saw an anecdote where Bill Spohn, who literally owns an HVAC tools company, had this happen with the contractor installing a system in his own house! (https://www.heatpumped.org/p/are-heat-pumps-a-commodity)

I suspect it's especially bad with new builds, as new builds are a race to the bottom and every subcontractor is fighting to get the lowest bid. The best way to make it cheaper is skip steps, and that hurts in the long run. Sorry you ended up in that situation, crummy experiences like this set the industry back. For what it's worth, the same corrosion could happen with a traditional AC system too (it's not just heat pumps). But the difference is, often those refrigerant lines don't get as hidden on interior walls as the ones for ductless mini-splits do.