Note: The other answer from LitExtension relates to Magento versions <= M 2.1, more recent versions do not use url_path attribute entries for URL rewrites. (Read more information in that answer for relevance - additional information related to this issue at the end of this answer as well).
This answer discusses the issue I solved eventually. My original ...
First check your affected rows with select query in url_rewrite
SUBSTRING(request_path, 1, CHAR_LENGTH(request_path) - 1) AS col_trimmed
WHERE request_path LIKE '%/'
Update affected row
UPDATE url_rewrite SET request_path = SUBSTRING(request_path, 1, CHAR_LENGTH(request_path) - 1) WHERE request_path LIKE '%/'