This link is invisible in any profile I post it in, whether an open or private profile:
https://www.ancestry.com/family-tree/person/tree/18668044/person/140143604249
I also tried these variants, also invisible:
[https://www.ancestry.com/family-tree/person/tree/18668044/person/140143604249 John Miller]
[https://www.ancestry.com/family-tree/person/tree/18668044/person/140143604249 1]
I think this might be another example of the ad-blocker hiding Ancestry urls. I tested one of your links on one of the profiles I manage and it displayed correctly, but I don't have an ad-blocker installed.
Have you tried using {{Ancestry Tree|18668044|140143604249}} instead?
As Paul indicates, the use of Ancestry external link templates currently avoids the problem of ad-blockers hiding Ancestry.com links in a profile. For links to pages on Ancestry.com for which WikiTree has no external link template available, such as a link to a publicly sharable Ancestry image (viewable by non-subscribers), I've had success in avoiding ad-blocked hidden links in profiles by prefixing the links with WikiTree's affiliate link as in this example. Some examples of coding for Ancestry links and the resulting displays can be found on this Ancestry Link Test Space Page.