Warnings only get triggered when previous `_build` folder is deleted

[umaucher]

When playing around with dependencies and needs warnings triggered for a run are not shown consistently.

Steps to reproduce (sample usage in https://github.com/eclipse-score/persistency):

# (valid) build of docs target:
$ bazel run -- //:docs

# remove lines which should trigger a warning
$ git diff
diff --git a/BUILD b/BUILD
index 8772102..9b5957b 100644
--- a/BUILD
+++ b/BUILD
@@ -18,10 +18,6 @@ load("//:project_config.bzl", "PROJECT_CONFIG")
 # Creates all documentation targets:
 # - `:docs` for building documentation at build-time
 docs(
-    data = [
-        "@score_platform//:needs_json",
-        "@score_process//:needs_json",
-    ],
     source_dir = "docs",
)

# build docs target again but not trigger the expected warnings

the warnings are only visible if the previous created _build folder is deleted.

[a-zw]

I can replicate the bug in the docs-as-code repo too.

We do declare external_needs_source as impacting the whole environment (see add_config_value):

app.add_config_value("external_needs_source", "", rebuild="env")

Sphinx does recognize that such a change requires rebuilding all files. So why don't the warnings show up? Maybe there is some additional cache for the needs?