I've seen the exact same thing on a company that went from 5 to 50 employees in a similarly short time frame.

The issue happens if you start with a friend group without decent structures or leadership "because we are friends/anyway". This works if you got 5 people but it doesn't if you have 50 or 150. Because you don't just have friends who are enthusiastic about the mission there, but you have to fill the ranks with people who actually want to treat this like a job. Now the "bro" culture starts to fall apart.

With this size you start to get real issues at work that need to be handled with a correct structure, which you don't have because senior management still feels this is just a startup full of bros.

Bros don't mind working 60 or even 80h/week, every week, because of the mission. Employees do mind. So now you have a workload designed for 60h/week per employee that is shouldered by a 40h/week employee. So either they work 60h (probably without compensation for the overtime) or they cut corners and deliver crap quality.

Same with the way people interact with each other. Bros don't mind some rough jokes, but employees usually don't like it that much if their real concerns get brushed aside with the suggestion to maybe "calm your tits".

When going from startup to real company, you need to make big changes to the structure and work culture. If you don't, an LMG ensues.