With version 4.8, the behavior of search switch to Solr was like this:

• If the model define one content type at its root
  • If there is anoter field named contentTypes and it has a value, this value is used
  • If there are several values in the field named contentTypes, the contentTypes are in the query and the content type field is empty
  • Otherwise, the content type defined directly on the SearchModel is used
• If the model define several content types at its root, the switch was broken (repaired with recent in issues in recent versions)
• If the model don't define a content type at its root
  • If there is a field named contentTypes and it has a value, this value is used
  • If there are several values in the field named contentTypes, the contentTypes are in the query and the content type field is empty
  • Otherwise, the content type field is empty

With current version, switch to Solr search can supports multiple values in fields but it ignores the root content type(s).

Here is a behavior that could be nicer:

• Simple search:
  1. Get the value from the contentTypes field if exists and filled.
  2. Or get the value from the root contentTypes if defined.
  3. Or get all the final contentTypes (filtered if there are exclusion) from the root SearchModel.
• Advanced search:
  1. Get the value from the contentTypes field if it is at the root of the query and valid for all the query.
  2. Or get the value from the root contentTypes if defined.
  3. Or get all the final contentTypes (filtered if there are exclusion) from the root SearchModel.