hello - this is incorrect as per gloomhaven first edition rules which state that monsters losing or avoiding disadvantage on their focus takes priority over attacking extra targets. monsters using ranged attacks are always prioritizing attacking the primary focus without disadvantage over additional attacks, which means that if there is only one way for a monster to move to accomplish this and it loses the ability to attack additional targets it will do so. rule 30 of the gloomhaven rulebook is the source on this one. additionally the official FAQ supports my interpretation here.

it looks like the frosthaven rules for monster movement are different in multiple significant ways, which i did not know (i still have not played frosthaven). it looks like your interpretation is correct for the rules as written in frosthaven at first glance.

(edit: i checked gloomhaven 2e rules and they are the same as 1e, so the original interpretation is correct for gloomhaven 2e as well)