GA4 Client ID don't match the UA Client ID

Question

I am having Issues with matching my Client IDs on GA4, with client IDs in UA. In order to see my client ids in GA4 explorer, and in UA custom reports, I added in the GTM config tag custom dimensions for the client ID, session ID and timestamp.

In the GTM debugger, and also in the console, I see how the Client ID is matching for what is sent to UA and GA4, but when I see the Ids in respective interface, there is not a single Client ID that matches.

What could possible be wrong?

thanks

The Javascript variable for grabbing the Client ID is this: (I don't know JavaScript)

function() { 
   return {{CK - Client ID}}.slice(6);
}

Grabbing the client ID with a first party cookie, _ga