Once properly installed, Flarum has worked perfectly for me several years under Hestia, including numerous manual updates and extension additions using Composer via SSH.
However, the Quick Install for Flarum in Hestia is flawed. If I had to install from scratch, I would figure out how to set the document root in Hestia, then install manually via zip or Composer. Take time to understand the Flarum "Public" folder concept -- I personally found it incredibly confusing back then, and it turns out to be pretty simple. But don't ask me to explain it now because I don't remember!