Either UA made a mistake, or given this is apparently being seen across the Caribbean, likely a deliberate decision to price this way because they think they can get it - likely because they know demand is still going to be low, and so they are trying to extract max profit they can from the seats they do sell. They know most people traveling even in June are going to do so because of need, rather than want, so can price higher.
Also not sure the June schedule is fully set yet, and not even clear yet, AFAIK, whether the DR will even allow travel by June. UA might have no choice but to cancel flights. Flight decisions are being made on a daily basis (if not hourly).
if you need (that word being subjective, and being dependent on the government even allowing it) to travel to POP at that time, you have choices - buy B6 for the lower fare they are offering, buy UA (or another carrier) for the price they are offering, or wait if you think pricing will change in your favor. Know that no matter which option you choose, there is a good chance that flights may be canceled, govt restrictions still in effect, etc., and so be prepared for changes.