Field display styling options with Bootstrap 4

Using Boostrap 4 styling, I'm having some difficulties in applying Field display styling options.

{
 label: 'Office (<code>block</code>):',
 name: 'office',
'className': 'block'
},

for example, won't work. In the html, the block class will appear but with no effect. I tried to tweak the upper BS4 example by adding a 'block' class on one field just to try with no effect.

My setup is: datatables & editors from the generator with BS4 class.

Many thanks in advance.

Sébastien