Mini Split HVAC System Size for a 1700 Sq Ft basement,,,
18000 BTU = 5200W
5200W @240V= 21.67A

So, no, a 15A circuit is not sufficient. But, I've never heard of or seen a 15A/240V breaker. Seems to me they need would need to install a 30A/240V breaker and 10AWG copper circuit for an 18kBTU system... BUT, it is worse than that!

18kBTU isn't large enough for 1700 sq.ft. This site suggests 24kBTU for  1500 sq ft. (https://www.sogoodtobuy.com/blog/sizing-guide-for-a-mini-split-air-conditioner/).  Seems like your builder has, and continues, to grossly under size the system for your needs.