The Fix: Have this mod installed and Plated Leather Drifter's Pants below this mod in the mod order. Hope this helps!

Why (tl;dr at bottom): In FCS, the class that determines the material of the armor is set to GEAR_MEDIUM instead of GEAR_LIGHT. An armor's class, among other things, determines what modifier to give to armor; this made it easier to make different combinations of stats to make items. Also in FCS, the Plated Drifter's mod is listed as dependent on this mod. Turns out this mod fixes the change by turning the item's class from GEAR_MEDIUM back to GEAR_LIGHT.

Here is what I think happened: This mod came out a month after Plated Drifter's, and neither mod was updated after they first came out according to the upload and update dates on the front of this mod page. So what probably happened is that the creator was working on both mods, made Longer Drifter's Leather Pants change to GEAR_MEDIUM, realized it, then accidentally put in a fix in the Plated mod. This would automatically detect if Longer was present, make Plated a dependent, and then correct the fix without anyone noticing.

The reason I was the only one to notice the stealth modifier bug on the Longer Drifter's Leather Pants and then say something is because I didn't download the Plated mod; I download the Longer mod first because I saw it on the day Longer was trending on the Workshop; I downloaded Plated a few days ago. Most people with this mod downloaded the Plated mod earlier, and now most people download both mods at the same time, so they never notice the bug at all!

tl;dr The creator probably was working on both mods at the same time, made a mistake that gave the Longer item a stealth debuff, and posted the fix on Plated Drifter's Leather Pants before uploading it. Most people downloaded Plated first then this mod, so most people didn't even notice.

It was an interesting journey to look through FCS and discover the story behind the issue. I hope you enjoy the mod. Longer Drifter's Leather Pants is my favorite light armor pants tied with Samurai Clothpants. I also give Leather Pants to my tanks/heavy DPS people in early-to-mid-game before switching to other armor.