and you presumably want 'adatbázis', not 'adatbázi s'. There are always whitespace issues when mixing encoded words and plain text in one header. The proper encoding of that header is
Subject: PIVOT kutatásfinanszírozási adatbázis
Granted, various MUAs are more forgiving of the defects, but the Mailman issue is really in the underlying Python email package which insists that the terminating ?= be followed by white space in order that the encoded word be recognized as such.
Just to be clear, if I have a Q encoded word as in rfc2047, and there is a \n and space after it, mailman puts an extra space in place. The only solution is if I encode all "words" even if that is not needed. rfc2047 allow to have encoded words mixed with non encoded texts.
"Ordinary ASCII text and 'encoded-word's may appear together in the same header field. However, an 'encoded-word' that appears in a header field defined as '*text' MUST be separated from any adjacent 'encoded-word' or 'text' by 'linear-white-space'"
So this is a BUG in the underlying python package.