In the first week, the researchers tested 86 participants’ performance on recognition memory. They studied several dozen words, one at a time. later they reported in a subsequent test which words they’d seen.

During the fifth and final week, all participants repeated the original set of baseline tests and compared their performance to the results from the beginning.

All the participants showed improvement. But this improvement was no greater with the working memory training group, or worse in some fields compared with the control group.

This is just one study with a relatively small sample, so it shouldn’t be treated as decisive evidence.

Laura E. MatzenEmail; Michael C. Trumbo; “Practice makes imperfect: Working memory training can harm recognition memory performance”; Memory & Cognition
November 2016, Volume 44, Issue 8, pp 1168–1182