This is caused by the div block surrounding it, that should receive 'similar' traits in that case.

It is worsened, because in the case of stack in particular also takes fullwidth, due to the broken implementation of inline images with captions. These currently feature a figcaption that has display block, and no width constraint (shouldn't be allowed to be wider than the image frame, but it is).

This seems mostly fixed to me. Is that correct team ?

I checked in production some samples from https://en.wikipedia.org/wiki/Template:Stack:

{{stack |[[File:Test.png|thumb|right|Example 2a]] [[File:Test.png|thumb|right|Example 2b]]}}

Without float=left/right parameter - default is right. Is that what's been fixed?

If I remember correctly, it would be that the right floating stack, was opened in VE as full width and left aligned, pushing all visual adjacent content to be positioned underneath the stack template.

In T59296#1154512, @TheDJ wrote:

If I remember correctly, it would be that the right floating stack, was opened in VE as full width and left aligned, pushing all visual adjacent content to be positioned underneath the stack template.

Seems to be fixed to me - not happenning anymore: inserting Stack template(with/without specifying float- right) will position the template on the right.

Btw, left - puts the template on the left.