It seems to be something about CSS grid rules. I don't know what goes on here, to be honest; the actual rules are a bit of a mess and grid itself is not straightforward.
Quickest workaround is to use a userstyle with e.g. Stylem to remove the gird-template-columns rule from the .umb-block-grid__layout-container class.
Quickest workaround is to use a userstyle with e.g. Stylem to remove the gird-template-columns rule from the .umb-block-grid__layout-container class.