Olivier here, hi @ramcq and everyone, and sorry for being late to the party.
I wasn’t sure myself how this worked, but from the look of things electron-builder’s snap mode defaults to requesting connection to the browser-support interface, but with allow-sandbox set to false (see isBrowserSandboxAllowed(…) function).
allow-sandbox: true requires the snap publisher to be trusted for auto-connection to be allowed. This is typically the case for browser vendors, not for random electron app publishers.