Starting pay discrepancies- what do do?

I work at a large public institution and started at the institution three years ago. Earlier this year, I switched jobs in order to help advance my salary and grow my skillset. I tried to negotiate a higher pay than was offered when they offered me the job, but they weren't willing to budge. They cited equity amongst others with the same title as me in my unit as the reason; I could not get paid more than someone that had been there just over a year longer than me. I accepted the position because it was in the best interest for my career long-term.

Shortly after I started, another department within my unit hired someone with the same job title as me. The position has the same responsibilities as I do- all of us with our title have the same responsibilities. Public employee salary is public information, and this week we discovered that the newest hire started at a salary $11k more than me, and only $1k less than the most experienced person in our position (who has been in their position for well over a decade).

Needless to say, there are several of us that are not happy. I am not happy because I was told a couple of months earlier that there was no room for negotiation and the offer was firm, yet someone with a similar background and just a couple more years of time within our institution was able to get significantly more to start. I spoke with my boss, and its their boss that determines starting salaries. I let him know the situation, and if my pay was not adjusted, I would likely look to move on already. We have heard that they will not be adjusting anyone's pay, and we have not been given a rationale for paying the other new hire significantly more.

Given I have only been in the position for six months, I feel like it's too early to be applying to new jobs that would give me more money, but at this point, I feel like I was lied to and misled, and I don't know how I can continue working at my new job for much longer. I would feel ok if the starting pay was $2-3k different because the other new hire has more experience at our institution than I do, and was making a higher salary at their old job than I was at mine. But $11k is significant.

Have others experienced this? Is it too early to look for something different (meaning would that be a red flag for search committees)?