Keep in mind that people who make flamethrowers don't become asbestos. You need something to defend this assertion better. Kreia likewise was a high level user of Force Drain, yet she was defenseless against Nihilus.
Ye, it's a good point. At best I can throw a few indirect comparisons and logical assumptions.
For example, when Jaina electrocuted Kyp, he wasn't harmed and stated that as he used to produce it, so could absorb, although it was Force lightning, same logic could apply to Force drain. Complete mastery of something normally means that you not only able to use it but to defend against it as well like with swords you not only attack but block as well or Jedi use TK as well as shield them from it.
Ulic-Kel Droma had knowledge of how to resist Force drain. How did he learned it? Fact is that he used to be Sith himself, so it is very likely that he learned it by mastering Force drain himself.
Considering that Palpatine mastered all known darkside powers and is one of the most powerful Sith of all time, it would be pity of him to have such weakness. He was scholar and Rule of Two was all about passing teachings. After 1000 years of building up knowledge chances that the technique about defending against Force drain was skipped are way too thin.