A German keyboard or interface is probably not responsible for this. I'm in the U.S. and part of my brain is a calendar with the same date format and similar issues.
I put the calendar portion of the brain in a different file; instant activation and the search tool returned, or failed to return, in a similar fashion in that file as well. That second file is attached.
'2011' returns a large number of thoughts in Instant Activation, mostly but not exclusively from 2011-10 and 2011-11 and 2011-12 and the month thoughts. The Search tool returns 100 results, my maximum.
'2011-' returns pretty much the same, except the Instant Activation only shows a couple of the months, and the only dates before October are the last couple of days of September.
'2011-0' returns in Instant Activation about 2/3 of the days in September, and one day in January. A search can not find any thoughts.
'2011-01' returns in Instant Activation one day - '2011-01-10 Monday'. The search tool finds all 32 thoughts (every day plus the month).
'2011-02' and '2011-03' and '2011-04' are identical in effect - Instant Activation can't find any matching thoughts, but the search tool finds every one.
'2011-1' returns in Instant Activation what looks to be every day in October, November and December. I'm not sure as they are listed out of order. Two of the month thoughts show up (2011-11 and 2011-10), as do four days in September and one day in January (2011-09-19 through 16 and 2011-01-10). Search tool can not find any thoughts.
'2011-10' returns in Instant Activation what looks like every day in October plus one day in January and one in December. The search results are correct.
'2011-11' and '2011-12' are similar to '2011-10'.