The changelog for Seventh Release (2007-July-03) states this "Corrected the tome description and the magnitude of Fenrik's Welcome to open average lock as it is a Journeyman-level spell, yet was advertised to open hard locks, but would not open most of them"
Oblivion Remastered does the following "Due to how Open spells function, Fenrick's Welcome can open any door up to 50 points. As a result the spell can open certain Hard locks, despite being labeled as an Open Average Lock spell. Oblivion Remastered This issue has been addressed in Oblivion Remastered; the spell now has a magnitude of 80, ensuring it opens any Hard lock"
Maybe the remastered fix should be included. You can find mention of the remastered fix here
https://en.uesp.net/wiki/Oblivion:Spell_Tomes