Possible problem: "Search API Solr" module 8.x-2.7 does not support Solr 8.
Remove "Search API Solr" module 8.x-2.7 using 'composer remove drupal/search_api_solr'
Install "Search API Solr" module 8.x-3.1 using 'composer require drupal/search_api_solr'
Find Solr modules in Drupal BO ("Extend" > "List") and click "Install" button
Add Solr Server in ...
I solved it myself (that's a first and probably a last)!
I'm sure this isn't the best code but it works and I don't know enough PHP to clean it up.
I disabled all "No Results Behaviors" in the view settings. I'm using the hook_preprocess_views_view hook to count the number of results for each of my attachments. Only if all 4 === 0 will it print the correct ...