Setting the following to 100% max-width instead of max 600px fixes it on larger screens. We can then also remove the sidebar on smaller screens. No reason to have it max at 600px.
.acf-admin-single-field-group .acf-field-object-flexible-content .acf-is-subfields .acf-field-object .acf-label, .acf-admin-single-field-group .acf-field-object-flexible-content .acf-is-subfields .acf-field-object .acf-input
{
max-width: 100%;
}