Delimiters, which are punctuation or spaces on either side of a string of characters, are used generate search terms. For example, a message which contains

123.456.987 000

will cause search terms to be indexed for "123", "456" and "987". For the purposes of indexing, neither the type of punctuation used (comma, period, question mark, etc) nor its placement matters. Likewise, the search process is agnostic to the punctuation used, so searching on any of the following will find the example message above:

456.987
456!987.
"456!987."

Terms of one or two characters are not indexed. The minimum term that Thunderbird will index is three characters. So, a message containing the following will not have "4" nor "91" indexed:

4.91.456.987

As a result, attempts to search on "4" or "91" will not find any messages. Also, adding a short string like "22" or "32" to other search terms will not affect the search results - these short strings are in effect ignored. "22 456" will find the example message above, even though "22" is not in the message.

Documented at https://support.mozilla.org/en-US/kb/global-search

KB is your friend.

So to sum up, it simply does not work and i cannot search emails for info about past SW versions?

Consider the add-on "Expression Search - NG", which can do more complex searches, including the use of regular expressions.

https://addons.thunderbird.net/en-US/thunderbird/addon/expression-search-ng/

A completely new and more robust backend storage and indexing is being worked on this year. Search on top of that will likely get attention next year, which will resolve your issues.