That is also a giant red flag. Normally, when you are paid via some non-taxable reward, it means your "promotion" isn't ever going to come with benefits that allow you to go climbing up the ladder. You made a good decision there.

My manager got a promotion with a hefty salary increase, then the company announced a hiring and salary freeze, then gave me a promotion with more responsibilities (some of my manager's as well) but with the same salary.

I quited a few months later as soon as I could secure something else

I'm dealing with that right now with my current company. They've had me working in a role 4 positions above the one my title would indicate for the last year. Being healthcare, they blamed not being able to change my job title because they can't afford increasing my pay during "these unprecedented times." Now, a year later, they're "working on updating my job title," but it's going to be a lateral title change with no change in payscale.
You bet I'm on my way out