While I agree that R180 音 is logically the correct radical, I note that that there are already very many cases where the traditional form uses the character below 𤇾 as the radical, whereas the corresponding simplified form uses the grass radical (R140). For example, in the URO we have: 塋 (32.10), 煢 (86.9), 榮 (75.10), 滎 (85.10), 犖 (93.10), 熒 (86.10), 瑩 (96.10), 螢 (142.10), 縈 (120.10), 鎣 (167.10); but 茔 (140.5), 茕 (140.5), 荣 (140.6), 荥 (140.6), 荦 (140.6), 荧 (140.6), 莹 (140.7), 萤 (140.8), 萦 (140.8), 蓥 (140.11). As there is a tradition of classifying 𫇦 characters under R140, it is OK to keep UTC-00740 as R140, but we could add R180 音 as a secondary radical (if so, then add secondary radicals for all other existing ⿱𫇦X characters classified under R140).

史式:《太平天国词语汇释》（四川人民出版社, 1984年）p. 437:

Evidence for the original glyph shape (⿰耳人) from 史式:《太平天国词语汇释》（四川人民出版社, 1984年）p. 257:

▲ SJ/T 11239—2001, 28-46

《京本全像插增田虎王慶忠義水滸全傳》卷之九 (第41回):



It seems that there is sufficient evidence to encode ⿰彳孟, but unfortunately it was not included in China's submission for WS2024.