Twenty senators did not turn up to vote on the constitutional amendment to lower the voting age to 18 on Thursday.
Of these, 11 were from BN, one from PAS, and four from former BN-affiliated parties.
There was also one senator missing each from Pakatan Harapan component parties PKR, Bersatu and Amanah as well as the ruling coalition's ally Upko.
The absent Amanah senator, Aiman Athirah Al Jundi, is on leave to perform the Hajj.
Bersatu senator Khairudin Abd Samad informed Malaysiakini that fellow Bersatu senator Isa Ab Hamid is also performing the Hajj.
The absentee BN senators account for 40.7 percent of the coalition's 27 Dewan Negara members.
Among the notable senators missing were MCA vice-president Ti Lian Ker, former Umno Youth vice-chief Khairul Azwan Harun, and Senate Reform Working Committee chairperson Mohd Yusmadi Mohd Yusoff from PKR.
Khairudin said there were 63 senators in attendance during the morning session that day, but only 48, including the Dewan Negara president SA Vigneswaran, were left after the lunch break.
Of the 68 senators in the upper house, Harapan only has 26 senators and three more of allied parties. However, it has been getting enough support in the Dewan Negara to pass laws.
On July 16, the Dewan Rakyat passed a bill to amend the Federal Constitution to lower the voting age to 18.
The bill was passed by a vote of 211. There were no abstentions or nays.
This is the first time the constitution has been amended since the Abdullah Ahmad Badawi administration.
In April, Harapan failed in its first attempt at a constitutional amendment, which involved restoring the status of Sabah and Sarawak as equal partners with Peninsular Malaysia.