This run 
https://uberchromegw.corp.google.com/i/chromium.gatekeeper/builders/Chromium%20Gatekeeper/builds/1223057
should have picked up failed build 90 
https://ci.chromium.org/p/infra/builders/luci.infra.ci/infra-continuous-recipes-tests/90
after green build 89
https://ci.chromium.org/p/infra/builders/luci.infra.ci/infra-continuous-recipes-tests/89
and presumably closed the tree. But it didn't happen.

Gatekeeper log says:
https://logs.chromium.org/v/?s=chromium%2Fbb%2Fchromium.gatekeeper%2FChromium_Gatekeeper%2F1223057%2F%2B%2Frecipes%2Fsteps%2Fgatekeeper%3A_infra%2F0%2Fstdout

loading build_db from infra_db.json
INFO:oauth2client.client:Refreshing access_token
INFO:oauth2client.client:Attempting refresh to obtain initial access_token
INFO:oauth2client.client:Refreshing access_token
INFO:root:fetching https://luci-milo.appspot.com/prpc/milo.Buildbot/GetCompressedMasterJSON with {"exclude_deprecated": true, "name": "chromium.infra"}
WARNING:oauth2client.util:new_request() takes at most 1 positional argument (2 given)
INFO:oauth2client.client:Refreshing access_token
INFO:oauth2client.client:Attempting refresh to obtain initial access_token
INFO:oauth2client.client:Refreshing access_token
INFO:oauth2client.client:Refreshing access_token
INFO:oauth2client.client:Attempting refresh to obtain initial access_token
INFO:oauth2client.client:Refreshing access_token
INFO:oauth2client.client:Refreshing access_token
INFO:oauth2client.client:Attempting refresh to obtain initial access_token
INFO:oauth2client.client:Refreshing access_token
INFO:root:fetching https://luci-milo.appspot.com/prpc/milo.Buildbot/GetBuildbotBuildJSON with {"build_num": 12541, "exclude_deprecated": true, "builder": "infra-continuous-precise-64", "master": "chromium.infra"}
WARNING:oauth2client.util:new_request() takes at most 1 positional argument (2 given)
INFO:root:fetching https://luci-milo.appspot.com/prpc/milo.Buildbot/GetBuildbotBuildJSON with {"build_num": 90, "exclude_deprecated": true, "builder": "infra-continuous-recipes-tests", "master": "chromium.infra"}
WARNING:oauth2client.util:new_request() takes at most 1 positional argument (2 given)
INFO:root:fetching https://luci-milo.appspot.com/prpc/milo.Buildbot/GetBuildbotBuildJSON with {"build_num": 12586, "exclude_deprecated": true, "builder": "infra-continuous-trusty-64", "master": "chromium.infra"}
WARNING:oauth2client.util:new_request() takes at most 1 positional argument (2 given)
INFO:root:no tree-closing failures!
saving build_db to infra_db.json