The "SSL handshake failed: Unable to get local issuer certificate" error occurs because the root certificate required to validate the SSL certificate is missing, so a system related issue and not a bug in Strawberry.
Flatpak sandbox is strictly isolated and does not automatically read SSL/TLS certificates from the host system's trust store, it's probably a bug in the runtime, missing the root certificates, could be this bug: https://invent.kde.org/packaging/flatpak-kde-runtime/-/work_items/91