The issue likely stems from differences in environment variables, permissions, or network settings between running Bitcoin Core (BC) manually and via systemd. When run manually, BC inherits your user's environment, while systemd provides a minimal environment. Ensure that necessary environment variables (like HOME, BITCOIN_DATA, and PATH) are set explicitly in the systemd unit file. Also, check that the user running BC under systemd has the correct permissions for the Bitcoin data directory and files.