Highlighting boolean queries shouldn't always highlight some clauses

I am having difficulties with getting correct "highlighting" section from Solr.

My query returns correct results, only highlighting does not work as I would expected.

My query:

http://solrServer/solr/solrCore/select?q=(((field1:((word1)AND(word2)))%20OR%20(field2:((word1)AND(word2)))%20OR%20(field3:((word1)AND(word2)))%20OR%20(field4:((word1)AND(word2)))))&fq=field5:(xxxx)&hl=true&hl.q=field1:(word1)&hl.fl=field1,field2,field3,field4

If I run this query the highlighting section is correct - there is no document with phrase "word1" - therefore field1 is not listed in the highlighting element - correct.

If I update my query to:

http://solrServer/solr/solrCore/select?q=(((field1:((word1)AND(word2)))%20OR%20(field2:((word1)AND(word2)))%20OR%20(field3:((word1)AND(word2)))%20OR%20(field4:((word1)AND(word2)))))&fq=field5:(xxxx)&hl=true&hl.q=field1:(word1 OR word2)&hl.fl=field1,field2,field3,field4

then I am not getting expected results, word2 has been found in field1 but word1 is missing, but Solr returned field1 in highlighting element with highlighted "word2" only.

I have explicitly added an extra query using hl.q and I have used AND operator (word1 AND word2), why Solr returns field1 in case when only word2 has been found?