Under construction pages in matrix can only be viewed by a logged in user.
To index these in Funnelback you'll need to crawl your Matrix site as an authenticated user (that has read permissions to view the pages you expect to see in the search results).
When configuring authentication with Matrix you normally need to configure form authentication - with something like this in your data source configuration:
crawler.form_interaction.pre_crawl.matrixlogin.url: http//<MATRIX-SERVER>/home
crawler.form_interaction.pre_crawl.matrixlogin.form_number: 1
crawler.form_interaction.pre_crawl.matrixlogin.cleartext.sq_username: <USER-NAME>
crawler.form_interaction.pre_crawl.matrixlogin.encrypted.password: <PASSWORD>
HTTP Basic authentication will only work in Matrix if that has explicitly been configured on the server (and in the DXP this probably isn't an option).
Don't forget to remove this form interaction configuration once your site is live though otherwise you'll continue to index unpublished content.
Note also that you'll index anything that the configured user can see, so if you personalise the page for that user those personalisation will be included in the index.
There's a bit more information on form interaction here in the documentation: https://docs.squiz.net/funnelback/docs/latest/build/data-sources/web/form-interaction.html