It's a quite normal situation. For example, when I started working in NLP, I did some things with the CYK algorithm (https://en.wikipedia.org/wiki/CYK_algorith

It's a quite normal situation. For example, when I started working in NLP, I did some things with the CYK algorithm (https://en.wikipedia.org/wiki/CYK_algorithm https://en.wikipedia.org/wiki/CYK_algorithm), which was defined in three papers by three separate authors. While I learned the algorithm from secondary sources, I of course cited the original papers (because one has to acknowledge the original author of the algorithms one uses). At that point, I wanted to read the original papers, even if just for curiosity, but they were impossible to find (I think one of them was available, but behind an outrageous paywall). Now those papers are easier to find, but anyway I don't think I'd encourage a grad student to read them except for curiosity, the algorithm has been explained much better and in more accessible ways in more modern papers. Which doesn't mean that we shouldn't acknowledge the discoverers.