Communication between team members is a huge issue in remote work.
It works if everyone is on site. It also works if everyone is remote.
But when some are at the office and some aren't, communication and information sharing becomes a lot harder. Mostly it's a process and tool issue, but still humans will rather just turn around and ask the team than spend a minute writing their issue on Slack/whatever.
It's true. I worked at a company that was 100% remote when it started and kept the remote culture as it grew from 4 to 25 people, but one city in particular ended up having enough people that they decided to rent space in a big shared office, coworking-style. Ultimately, people who worked in that space ended up being more connected to what's going on, due to physical proximity. You wander past a hallway conversation and you end up joining, or at least knowing about it. You go to lunch with your coworkers and happen to get a little work talk in. You ask questions more easily from someone who's right across the desk from you because you can read social cues about how "interruptible" they are. Jokes come across better in person than via messaging. There are lots of subtle ways in which in-person interaction is just inherently different than remote, and the office people end up tighter.
It's not even remote vs non-remote. My employer had many medium sized offices of 100-200 people, and we merged with a company that had one single 5000+ person campus. That was a major culture clash for sure in terms of documentation and processes
But it's not actually hard, managers just aren't trained on how to deal with the split. Leadership can demonstrate how to manage both, and if there are still problems, you can just mix up the office seating.
Even in remote-heavy orgs, communication issues remain if the culture or process is shit. If you have 10 different Jira sites, 4 forms of communication, and documents in 15 different systems, it's going to suck no matter where people work.
It works if everyone is on site. It also works if everyone is remote.
But when some are at the office and some aren't, communication and information sharing becomes a lot harder. Mostly it's a process and tool issue, but still humans will rather just turn around and ask the team than spend a minute writing their issue on Slack/whatever.