Could you elaborate on the steps to reproduce by providing a specific example / testpage?

You can test it with every page.

Anyway, let's take one to make an example: https://it.wikivoyage.org/wiki/Caraibi

1. Edit it.
2. Change (in the very bottom):

{{Quickfooter|Regione continentale

}}

into

{{Quickfooter|Regione continentale

}}

3. Preview it.
4. You'll see that in the top, instead of

America Centrale > Caraibi

you'll keep seeing

America del Nord > Caraibi

5. but if you save it, you'll see the correct layout: America Centrale > Caraibi

Let me know if now it's clear or if you need more info about it.

I still see it as unconfirmed. Do you need further clarification?

I can confirm the problem with the given steps, however I am not sure if this is a bug in the MediaWiki code itself (which would make this a valid, confirmed bug) or in the {{Quickfooter}} template.

Quickfooter is a template present only in it:voy (e.g. en:voy use IsPartOf), but at the end it just use the tag {{#isin:...}} like all the others language versions of voy.
Is this tag that originate the issue.

Simpler steps to reproduce:

1. Edit a title in namespace 0, for instance a new page
2. Add {{#isin:America del Nord}}, preview

I. Observed: breadcrumb is not shown.
II. Expected: it's part of preview.

3. Save.

III. Observed: it's shown.

4. Edit again, replace with {{#isin:America Centrale}, preview

IV. Observed: the old breadcrumb is shown.
V. Expected: it's updated.

5. Save

VI. Observed: it's updated correctly despite preview.

Change 299926 had a related patch set uploaded (by Legoktm):
Update breadcrumb properly on preview

https://gerrit.wikimedia.org/r/299926

Change 299926 merged by jenkins-bot:
Update breadcrumb properly on preview

https://gerrit.wikimedia.org/r/299926