Pre-images and change collection tables prune old content using untimestamped truncates. A read establishing a snapshot at a point in time (PIT) may see data inconsistent with that PIT: data that should have been present at that PIT will be missing if it was truncated, since a non-truncated operation effectively overwrites history.

This behaviour is similar to capped collections before they replicated deletes explicitly (i.e. before SERVER-16049); we similarly banned reads with snapshot read concern on those (see SERVER-47574 and SERVER-47824).