The terminology is confusing. 'Applying' to Deltas is not the same as 'Applying'
'Applying' transforms is baking object transforms into the mesh. 'Applying' to Deltas is transferring object-level transforms to a secondary transform matrix. Really, the Delta transforms are useful only in animation, when you want to keep a zero-point handy, to return to.
When you 'Apply to Deltas', the object is still carrying an object-level transform, just in a different place. The 3 Bevels below are: 1. with the object carrying a non-uniform scale, 2. with the scale 'Applied to Delta', 3. with the scale truly 'Applied'.

Bevelling before Applying object-level scale is the same thing as scaling after bevelling. That's the internal order of events. There really is never any need to scale in Object Mode during modelling, except, perhaps, when setting 'Dimensions', in that panel. Avoid, whenever possible, the most common cause of questions on BSE. :)