Paper finding that explicitly welcomed minorities felt less welcome
I think the description provided (if accurate) should be enough to find the paper, unless it is not available online, or is only available in a different language.  Sometimes authors of books and certain review papers will reference unpublished research, so it may have come up in that context.  Or it may not exist at all.

The results described are in line with what diversity training guides often recommend, for example:


  Do talk about everybody’s differences. Teaching about cultural differences must be inclusive of all people, at all times.
  
  Don’t single out particular groups, such as ‘ethnic groups’.


Unfortunately, such materials rarely provide references to support recommendations.

This area of research falls under "multiculturalism vs colorblindness" - the question of whether it is best to highlight differences between groups, or to ignore them.  A recent review (2018), that also references 3 other recent comprehensive reviews, echoes the findings of over 25 years of research (eg, see earlier review (1993)) that the multicultural approach is generally superior across a wide variety of outcome measures.

That said, there are certainly many ways in which attempts to foster diversity can go wrong, and can be more clearly refined.  Perhaps singling out specific groups, as suggested by some training guides, is one of those pitfalls (for a similar example, see the minority spotlight effect).  It would be interesting to see a study that addresses this question directly.