The problem was that the modifier was reevaluated all the time, even between showing the attribute search and clicking on the attribute name. This freed the data referenced by attribute search. The real bug here was that the dependency graph was tagged for update even though nothing changed. This was because the spreadsheet thought its active context has changed and it wanted to compute the new value to be shown in the spreadsheet.

The reason for the bug was that I confused how the tree-path of a node editor works. The second element in the tree path contains the name of the group node in the root tree that we're in (instead of the first element).