Shalom Aleichem Rebbi UMori.

Rashi to Niddah 49b (s.v. Rabbi Yehuda) explains that R' Yehuda rejects the method (for checking if an earthenware vessel's hole is large enough to allow seapage in) suggested by the Sages because their method might lead to a false positive. In other words, on account of pressing the earthenware vessel into a bath of water, water might leak in even though the hole is not halakhically big enough to be "machnis". Therefore R' Yehuda suggests a different technique - inverting the earthenware vessel upside down, and then filling a surrounding bath with water.

My question is as follows: Rashi describes the false positive that R' Yehuda is afraid of receiving as a "kula". However, quite the contrary - it seems that a false positive would create a chumra. After all, if a false positive is received, we will assume that the vessel is disqualified for sanctification of parah adumah waters. This is a stringency and not a leniency! (Rashi on 49a, s.v. "Shi'uro kichoneis mashkeh", emphasizes that this is the only nafka minah of having a hole big enough to be machnis.)

Thank you very kindly.