It resets for me too. I’m not sure if it’s a bug, or intended.
But it works in the Sandbox vault.
I tried deleting my .obsidian folder (after saving a copy). I disabled all snippets. I’m on default theme. And it still resets each time.
What OS are you on? I’m on MacOS like that other thread.
Alternatively, instead of setting zoom, you could use a CSS snippet and permanently adjust the style of all text to be bigger. Maybe it would even be possible to detect which system you are on, so it won’t affect mobile, for example. (If you sync.)