I also think. revalidateTag seems to be inconsistent to me in both cases. The documentation on revalidation on demand itself contradicts with revalidateTag and revalidatePath. :/

Try upgrading to the latest version of Next.js and try again? Some of the problems should have fixed in the new updates.

There are several issues on their Github repo related to revalidateTag. Currently, revalidatePath works great for my blog though.
Like https://github.com/vercel/next.js/issues/49509 and https://github.com/vercel/next.js/issues/55960
If you found their documentation is inconstant, feel free to raise a PR or open an issue.

I am already using the last stable version. I am also aware of these open issues. I haven't opened any issues yet because I found several questions about revalidateTag and revalidatePath problems, so I think it's better to wait for some answers from these issues so as not to have redundancy.

Note: Today I deleted the .next folder and executed the project again, and incredibly revalidateTag worked as expected. During some tests, I could identify that during some access to the page, one data or another (rarely, but it happens) ends up not being the latest. This is really very strange.

Yeah that's happening sometimes even in the latest version of Next.js. revalidatePath works though

.next might have cached some files, better deleting it after upgrade to a new version