Missing url data in log_action and log_link_visit_action


I am analyzing the frequency and the content of url data from a Piwik MySQL dump file.
The tables log_link_visit_action and log_action are of my interest.

From a specific time upon the variable idaction_url in the table log_link_visit_action and the table log_action is not filled anymore in MOST cases.
This seems to be due to a custom variable, that has been implemented at the same time as the described behavior occurs.
This custom variable was implemented to count and analyze searches users perform on the website. This means that each time a user performs a search, the custom variable shows an ID but the idaction_url field is NULL an the url doesn’t get listed in the table log_action as well.

I really need the information the search-urls contain. The information from the custom variable doesn’t help, so I need the complete url string.

I didn’t implemented this Piwik installation and I have no further experience with Piwik administration.

Do you have any idea why the information is missing? Do you know, if the missing information is stored somewhere else in the dump file?
I am happy for every advise to understand the problem.

Thank you very much in advance,