Potions, Splash Potions, Lingering Potions, and Tipped Arrows all cannot be given a name using the "item_name" component.
Steps to Reproduce
1./give @s potion[minecraft:item_name=customtext]
2.Notice that the item is not called "customtext"
MC-279710
Potion Names not overriden by "item_name" component
Resolved
Works As Intended
0
WAI per https://bugs.mojang.com/browse/MC-276327?focusedId=1364564&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-1364564, see also MC-278190 for a related problem whose fix would solve your problem.