Solr has a limit on the number of facets - which has nothing to do with the
Query.MaxItems btw (that is for Sitecore queries). You should be able to set
facet.limit to -1 to get all results.
Not sure if in the current implementation by Sitecore there is another solution possible already, but this was given at the community site answer mentioned above:
On your Solr Server, edit the solrconfig.xml for the index you are
using. It is found in the conf folder of your index.
Find the requesthandler you are using, for instance "/select", and add
-1. Be aware that you have to do this
for each index that you want to update.
For me it looks like this after the fix:
<requestHandler name="/select" class="solr.SearchHandler">
<!-- default values for query parameters can be specified, these will be
overridden by parameters in the request -->
One more blog with the same solution explained: https://blog.ryanbailey.co.nz/2018/07/sitecore-experience-commerce-facets-are.html