special characters in FTS Solr fail

[piuma at piumalab.org]

Hi, I want to report a bug about searching.
My dovecot version is 2.3.10.1 (a3d0e1171) and use Solr 7.7.2 for full text search.
The issue appear when try to search using solr special characters. For example
if I have emails with the subject that contain the only character "(" open
parenthesis, then try to search it I receive zero results.

In the Solr log seems the "(" is correctly excaped as "\(":

2021-03-30 15:14:20.304 INFO  (qtp333683827-20) [   x:dovecot] o.a.s.c.S.Request
[dovecot]  webapp=/solr path=/select
params={q={!lucene+q.op%3DAND}subject:\(&fl=uid,score&sort=uid+asc&fq=%2Bbox:6fff5821c5a2365df4060000d0d97f83+%2Buser:titolare01 at kendama.it&rows=631&wt=xml}
hits=0 status=0 QTime=2

I provide dovecot-sysreport as attachment.

Best regards
   Danilo
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