When a URL carries a noindex tag but also appears in your sitemap, the two are giving search engines opposite instructions, and the noindex wins for the question of indexing while the conflict quietly damages everything else. The sitemap entry says “this page is worth indexing, please find it,” and the page itself says “do not index me.” Search engines resolve that contradiction the safe way: they respect the noindex and keep the page out of the index. So your indexing outcome is correct, but the signal you sent was muddled.
The cost is in clarity, not in the index itself. A sitemap works because it is a confident, consistent statement of which pages you want found. Listing a noindexed URL breaks that consistency. You are simultaneously inviting search engines to index a page and forbidding it, which wastes a slot in the file you built to communicate intent. On top of that, reporting tools tend to flag the conflict, surfacing the URL as a noindexed page submitted in a sitemap, which adds noise to your coverage reports and can mask real problems behind a self-inflicted warning.
The mechanism, then, is that the page-level directive overrides the sitemap suggestion for indexing, but the mismatch itself is the damage: it dilutes the sitemap’s reliability and clutters your diagnostics with avoidable conflicts. There is no scenario where keeping a noindexed URL in the sitemap helps you, since the page will not be indexed regardless. The fix is consistency. If a page should not be indexed, it should not be advertised in the sitemap, and if it belongs in the sitemap, it should not carry noindex.
To clean this up, cross-reference your sitemap against your noindexed pages and remove every noindexed URL from the file. Keep the sitemap to canonical, indexable pages only, so it speaks with one voice and your coverage reports stop flagging contradictions you created yourself.