In "Per-user multifactor authentication" I disabled MFA for one user; All got disabled

The behavior you encountered is a recognized limitation of the legacy Per-user MFA management page in Microsoft 365. Although the interface allows you to select an individual account, the “Disable MFA” action may inadvertently affect all users in the tenant. This occurs because per-user MFA is an older mechanism that does not consistently function as expected. Microsoft now advises administrators to manage multifactor authentication through Conditional Access policies, which provide more reliable, granular, and supported controls than the per-user portal.