Interestingly this one seems it is from before 高輪ゲートウェイ (Takanawa Gateway) station which opened in 2020, but the numbering shows the gap (JY 25 -> JY 27). That led me to looking it up, and turns out that they introduced the numbering in 2016, and that already came pre-planned with the gap ready [1].
[1] https://www.jreast.co.jp/press/2016/20160402.pdf