However, acting Youth chief Kennedy John Jie said the movement would carry on with its goals to support the party in all its endeavours.
In a press conference held at the party’s headquarters here tonight, Jie said the party did not expect Sen’s sudden decision, especially after all the good things Sen had done for the party.
“We last saw each other officially in April during a meeting for the youth wing. After the general election, we have not had any meetings yet.
“Nevertheless, we wish him all the best and good luck,” he said.
Jie admitted that he had not spoken to Sen personally and even those close to Sen said they did not know the reason behind Sen’s resignation.
Sen has said he wished to keep the reasons to himself for now and did not say whether he would join another party.
Jie said Upko youths were still adjusting to being in a new coalition with former political nemesis Parti Warisan Sabah.
“It takes time to adjust but we are coping,” he said.
Upko was one of four local Sabah parties in the BN coalition before the 14th general election.
After BN and Warisan-PH were tied with 29 seats each, Upko acting president Madius Tangau decided to bring four of the five Upko assemblymen out of BN in favour of Warisan-PH.
Upko is one of three Kadazandusun Murut majority parties in Sabah. The others are Parti Bersatu Sabah (PBS) and Parti Bersatu Rakyat Sabah (PBRS).