As a maintainer of edit filters, I expect variables such as added_lines to be present among the "Variables generated for this change" for every edit action, since the documentation provided at mw:Extension:AbuseFilter/Rules format#All variables does not mention any case where it is expected that the added_lines (or any other variable) will be missing for an edit action.
However, for some reason it is not present at w:pt:Special:AbuseFilter/examine/log/3736279 (while it was present in a log prior to that), so I guess some recent change to MediaWiki or AbuseFilter introduced a bug in the software. In fact, all the following variables are missing:
user_editcount, user_age, user_groups, article_recent_contributors, edit_diff, new_size, old_size, edit_delta, added_lines, removed_lines, added_links, all_links, old_links
The absence of these variables makes it difficult to investigate a possible false positive, and depending on the cause may also produce other false positives.