In my industry (Tech), it's not uncommon at all to have individual contributors earn more than front line managers.
It's also not uncommon to have newer employees (especially those hired in this current labor market), to have negotiated higher starting salaries than those of folks who have been employed for 5-10 years.
So I guess I don't have an answer to your question, but know it's not that uncommon.
It's also very common that women and people of color get lower salaries too.
The company - and the managers to some extent - have more information than individual employees and should safeguard equity in their staff.
experience and tenure do come with some justifiable salary increases, but elsewise management should be guarding against this.
I agree that manager sometimes, justifiably, make less than those they manage, but that is a relatively rare circumstance where the people you are managing have skill set that you do not, so while you oversee there work they are the ones with the creativity and skills to develop things you can't.
OP - your company sucks. This is a typical case where they company will take whatever professional growth you are capable of and profit form it without rewarding you.
I'd broach it with you manager today. You don't have to reveal that you compared with other just yet but you can have a conversation on your new role, new responsibilities, how you stepped up and what you are now contributing to the company (that is > what you did pre-promotion). You can ask about the timeline for an increase, what you need to demonstrate to get that increase, and just have the conversation very positive and focused on your desire to do well in the role and also for a good increase.
If they don't deliver, hit the job search hard on your 1-year mark int he promotion.