This has been reported at https://github.com/bitwarden/clients/issues/6894 and it seems to be specific to the Flathub version. The AppImage from bitwarden.com works well on high-resolution as of 2024-04.
This seems to be specific to the desktop environment. With a HiDPI monitor and using Wayland, the Bitwarden Flatpak interface appears blurry on GNOME Shell but crisp on KDE Plasma.
This has been reported at https://github.com/bitwarden/clients/issues/6894 and it seems to be specific to the Flathub version. The AppImage from bitwarden.com works well on high-resolution as of 2024-04.