Some Prometheus alerts mislabelled

Question

We've been using Prometheus with fairly solid success so far. However, we've recently noticed that one of the sets of alerts we've set up ends up mislabelled: It shows up as a different environment than it is. This happens with both environments where we have multiple targets, so I'm wondering if this is a syntax (somehow? The rest are fine) or (usage of) blackbox-exporter issue.

Specifically, environments with more than one target listed have the second and subsequent targets labelled as the next environment on the list (ENV-B1's .2 and .3 show as ENV-B2 while ENV-B2's .2 and .3 show as ENV-B3). The rest are correct.

These are my configs (with various bits of info replaced):

# prometheus.yml
rule_files:
- /etc/config/rules
- /etc/config/alerts
scrape_configs:
  # Note: Prometheus lists these in a different order, it's not clear why.
- job_name: "a client (direct)"
  metrics_path: /probe
  params:
    module: [https_health]
  static_configs:
  # ACME
  - targets: [ "http://1.1.1.1:8089/isAlive" ]
    labels:
      environment: S1
      project: ACME
      service: web-api
      importance: moderate
  - targets: [ "http://2.2.2.2:8089/isAlive" ]
    labels:
      environment: S2
      project: ACME
      service: web-api
      importance: moderate
  # B environments run multiple instances; Check all of them.
  - targets:
    - "http://3.3.3.1:8089/isAlive"
    - "http://3.3.3.2:8089/isAlive"
    - "http://3.3.3.3:8089/isAlive"
    labels:
      environment: B1
      project: ACME
      service: web-api
      importance: high
  # B environments run multiple instances; Check all of them.
  - targets:
    - "http://4.4.4.1:8089/isAlive"
    - "http://4.4.4.2:8089/isAlive"
    - "http://4.4.4.3:8089/isAlive"
    labels:
      environment: B2
      project: ACME
      service: web-api
      importance: high
  # B environments run multiple instances . . . except B3.
  - targets: [ "http://5.5.5.5:9089/isAlive" ]
    labels:
      environment: B3
      project: ACME
      service: web-api
      importance: high
# (more)
  relabel_configs:
  - source_labels: [__address__]
    target_label: __param_target
  - source_labels: [__param_target]
    target_label: instance
  - target_label: __address__
    replacement: 10.10.10.10:32222
# (etc)

# alertmanager.yml
global:
receivers:
- name: slack-devteam-acme
  slack_configs:
  - send_resolved: true
    api_url: "...url..."
    username: "Prometheus Monitoring"
    title: '{{if eq .Status "firing"}}:fire:{{else}}:successful:{{end}} {{.CommonAnnotations.summary}} {{if eq .Status "firing"}}failing{{else}}resolved{{end}}'
    title_link: "http://10.10.10.10:32600/alerts"
    text: "{{.CommonAnnotations.description}}\nInstance(s):{{range .Alerts}}\n{{.Labels.instance}} {{.Labels.environment}} {{.Labels.project}} {{.Labels.service}}{{ end }}"

# alerts.yml
- name: blackbox-export-basic
  rules:
  - alert: ProbeFailure
    expr: probe_success < 1
    for: 5m
    # labels:
    #   severity: page
    annotations:
      summary: "{{$labels.job}} probe"
      description: "{{$labels.job}} failed check for 5+ minutes."
      #description: "{{$labels.instance}} ({{$labels.job}}) failed check for 10+ minutes."

# blackbox-exporter.config.yml
modules:
  http_build_number:
    prober: http
    http:
      fail_if_body_not_matches_regexp:
      - "buildNumber"
  https_health:
    prober: http
    http:
      fail_if_body_not_matches_regexp:
      - "Success"
# (etc)