Solution:
The problem was that the Types custom field used to insert the tooltip text is a Wysiwyg field, which inserts HTML markup, and needs to be a simple text field that inserts plain text.
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Indeed when a static text is displayed, no problem occurs and it works perfectly.
The problem here is that the tooltip text is not static, it may change depending on the information in the view.
That's why I am trying to insert the value of the Custom Field in the Tooltip Text.
Yes, well it works fine in a normal View, so I will need to know more about the View you have a problem with.
Perhaps I should look at it on your site.
I will mark your next reply as private so that I can get log-in credentials from you—you may want to create a temporary admin user for me to use that you can later delete. And be sure to have a current backup of your site, even though I don't intend to break anything.
Can you also confirm which View we are talking about, and whatever I need to know about how you are displaying that View.
I managed to understand your set up and do some testing and I think the problem is that you are using a Types field for the title attribute which includes HTML and is not simple text, so it breaks the page markup.
If you abstract away from your complicated set up and simply tried to create a tooltip on a standalone page generating the title attribute using the same Types shortcode but setting a specific post (e.g. id="125") you will find it won't work.
You need to use a simple multiline text field for the description, not a WYSIWYG field.
The problem was exactly as you described it. Using a text field instead of WYSIWYG field fixed the problem. Thank you very much for your time and help.