tl;dr: mine were showing warning due to 2026 not being a valid tag; though it appears yours are all regular alphabet characters so must be a different issue
Fwiw I’m experiencing same warning on Windows, Obsidian version 1.12.7. I hadn’t updated my tags, finally went through and had the Linter plugin convert them, and they appear to be correct apart from this warning that will make no changes, and then if i switch to a different file and come back after “fixing it”, the warning will be back.
I pulled the “corrected” version out of source mode and did a diff, they appear to be exactly the same:
---
aliases: [2026 - W13]
tags:
- weekly_note
- 2026-W13
- 2026-03
- 2026-Q1
- 2026
---
Turns out the old style periodic notes of having a tag for just the year isn’t valid anymore. “Tags must contain at least one non-numerical character.”