fulltext search with wildcards and reserved characters is a difficult topic. At first, it’s important to understand how the term ‘EN 13859-2:2014’ is analyzed by indexing.
This term is broken into tokens at any non-letter character, such as numbers, spaces, hyphens and apostrophes, discards non-letter characters, and changes uppercase to lowercase. We use Introfind analyzer for search, it divides your term into EN, en, 13859-2, 138592, 13859, 2, 2014. Then it’s clear why some queries cannot return any hits.
It's correct that the two last queries 'EN 13859-2:20' and 'EN 13859-2:201' return no hits. To get results you need to use the wildcard at the end of the term. The first two should get hits. It’s a bug in the analyzer. We are going to fix it in the next version.
Another remark: allowing a wildcard at the beginning of a word (eg "*ing") is particularly heavy, because all terms in the index need to be examined, just in case they match.