During the merge process, it is not correctly displayed/documented that the length is not kept. Indeed, the length of (not standalone) recordings depends on the tracklist they are associated with. The merge recordings dialog currently does not correctly show the merge strategy for the length attribute in the (most frequent) case of not standalone recordings.
Fictive example: Screenshot attached, both recordings were added for a release (that is, not standalone), see lengths in the last column. If this merge is done, whatever the recording the merge is done into, the length will be arbitrarily inferred again from the tracklist times (probably to the shortest one).
For real example, see the editing history of the two following releases: