Does the president’s party always lose seats in midterms?

Almost always. In the 24 midterms since 1934, the president’s party gained House seats only 3 times: 1,934, 1,998 and 2,002.

The 1934 gain came during the Depression, 1998 during the Clinton impeachment fight and 2002 after the September 11 attacks. Those explanations are historians’ readings, not laws of politics.