(Preface: I'm just an undergraduate, so none of this actually matters; it just seemed an interesting case study.)
About years ago, a lab published a very unexpected result (in, I should add, a predatory journal).
Last year, another lab produced some evidence to support the paper (in, I should add, an even worse predatory journal, which doesn't matter except that they might not have heard of some of the issues in peer-review).
I failed to replicate the original results. There seem to be some subtle but serious issues with the original paper that might explain this; but I don't intend to publish this rough work. In particular, the method used in the original work is notorious for false positives.
I recently found out from funding reports that the other lab is still working on the phenomena and are probably done with their data-taking by now.
Is it even remotely good practice to send a quick email to the other lab with concerns?
(I wouldn't be concerned about bad work in C- journal, but this has some recent relevance.)