1. Have Release with Recordings with identical Relationships.
2. How to Use the Relationship Editor \ Batch-adding relationships: “One thing that might sound confusing is the message at the top of the dialog: "If the relationship already exists on one of the recordings, the attributes will be merged into the existing one." This is best understood by example: if there already existed vocal credits to Engelbert Humperdinck on tracks 1, 3, 5, and 9, and we used the batch-relationship dialog to add lead vocal credits, you would not end up with two vocal credits on these tracks. The relationship editor will modify the existing vocal credits by adding the "lead" attribute onto them.”
3. Batch edit according to How-to, adding Attribute.
4. Enter edit.

Expected result: Relationship updated with Attribute (in 1 edit).

Result: Relationship removed and added anew with attribute (2 edits).

From user perspective maybe rather point of display than bug, but in any case the How-to text (at least last sentence) is misleading.

Recent examples, both on Instrument:

Sextett nr. 2 G-dur, op. 36: II. Scherzo (Allegretto): Batch edit of ‘cello’ → ‘cello 1’ renders remove + add edits.

Concerto No. 1 en La majeur, op. 20 - 1. Allegro: Batch edit of ‘violin’ → ‘solo violin’ renders remove + add edits.