Revisions content disappear after sometime

Problem/Motivation

After at least one day revisions data disappear and there is an empty entity. After a longer period of time (about a year), revisions metadata disappear, for example the time only says "somewhen before".
In the database revision tables wisski__r__ the data seems to remain (not sure).

Steps to reproduce

Add a revision (edit existing entity -> add revision). Wait one day. Try to open the revision.