It might be because Google has not found the community subdomain yet. Specially if the community has just launched. The best way to introduce it is to add a sitemap to Google Search Console.
To add a sitemap, you should first add your community URL (e.g community.yourwebsite.com75) to the search console and verify your property.
If the property is not automatically verified, Google Search Console will give you a verification code. Go to Tribe Admin Panel, in the left menu select Apps, find SEO app and install it. Then click on Enable button in the app settings, enter the verification code under Google Search Console Verification Code and click update.
You can alternatively verify your property by adding a meta to <head> tag in community Admin panel under Theme > Advanced Settings.
After your property is verified, in the SEO app click on "Update Sitemap" button and copy the "Sitemap URL". Finally, go to https://search.google.com/search-console/sitemaps86 and add your sitemap URL.