It could just be that the value is being overwritten by activities on the subsequent pages (i.e., the pattern matching value is being set every time a page is processed, so it gets a value in the first one, but then that gets replaced with the blank value when it checks for the next page).

For example, you could create a token in your Pre-Classification processing to hold the value and use that to set your field value.

Then, in your pattern matching, store the pattern results to a token with a different name.

Lastly, add a Conditional in the Page Processing with an Assign Token Values under the conditional that assigns the pattern value to the main token you created.

For your condition, you could do "If Pattern Matching Token is not empty" or something along those lines, but the ultimate goal is to only change the "main" token when you actually get a value.