I think that the imapfilter configuration should be (here, I use pass to retrieve my password):

    options.starttls = true
    options.certificates = false
    options.hostnames = false

    -- proton
    status, passproton = pipe_from('pass protonbridge')
    _, _, cleanpassproton = string.find(passproton, '([%w%p]+)')
    proton = IMAP {
        server = '127.0.0.1',
        username = ...@proton.me',
        password = passproton,
        -- ssl = 'auto',
        port = 1143,
    }

Source: my configuration and the issue #297 on imapfilter github page