I don't think that's a real policy I don't think...? If you mean Luan Ming and Wen Li, both their designs and character were winning entries during a character
contest for timedomAIn's new voices
. They're timedomAIn's own voices though and the rules were laid out in the contest rules so you're not losing those characters unknowingly or through force (kinda like how VSinger had their characters, except Luan Ming and Wen Li keep their original lore and design)
If you mean Chu Ci and Que He, vocals from the HiDii Orchestra, I'm actually not sure. Que He especially moreso considering that his design was decided through a contest through ACE. There's nothing that suggests they're banned from moving to another engine though.
Aside from them, Yun Hao's coming to more engines after ACE (Vogen, Vocal Sharp and NNSVS are on his
profile page as upcoming voices). Hui Yu, an upcoming ACE vocal, as well I believe is coming to
more AI engines aside from ACE. Student Zhibin, another upcoming ACE vocal, will also receive a Vogen alongside an ACE according to his
profile page.