最后发现是需要设置PagerMode这个属性为PostBack
https://devnet.kentico.com/docs/12_0/api/html/P_CMS_UIControls_UIPager_PagerMode.htm
https://devnet.kentico.com/docs/12_0/api/html/T_CMS_DocumentEngine_Web_UI_UniPagerMode.htm
<cms:UniGridPager ID="pagerElem" ShortID="p" runat="server" PagerMode="Postback"/>
| Querystring | 0 | Querystring pager. | |
| PostBack | 1 | Postback pager. |