SOLR query runs OK only on 1 node out of the 3 nodes with SOLR installed - solr

Question

Here's our problem. We're runnig DSE Enterprise 4.8 with the following configuration:
6 servers with Cassandra
on 3 of them Spark is installes (DSE Analytics)
on the orher 3 SOLR is installed (DSE Search)
We want to do "query stats" with SOLR on a table using and excluding certain filter queries.
When we try to execute a simple query like this:
/select?q=*:*&wt=json&indent=true&fq={!tag=fq1}test:100&stats=true&stats.field={!ex=fq1}test&rows=1
it runs OK only on 1 node out of the 3 with SOLR installed.
For the other 2 nodes we get this exception:
{
"responseHeader":{
"status":400,
"QTime":2},
"error":{
"msg":"undefined field: \"{!ex=fq1}test\"",
"code":400},
"params":"q=*:*&indent=true&stats=true&fq={!tag%3Dfq1}test:100&rows=1&wt=json&stats.field={!ex%3Dfq1}test"}
Could you help us identify the reason of the "undefined field" exception when using exluding filter queries inside the stats.field parameter.
Also it would help us to use only a subset of the stats functions ( for example only count ) :
stats.field={!count=true}test
But these types of parameters seem to be ignored and the whole set of stats functions are computed...
Many Thanks