ES+1 (7.10.0 snapshot) doesn't work on Kibana 7.9.0

[bhavyarm]

Kibana version: 7.9.0 snapshot

Elasticsearch version: 7.10.0 snapshot

Server OS version: darwin_x86_64

Browser version: chrome latest

Browser OS version: OS X

Original install method (e.g. download page, yum, from source, etc.): from snapshots

Describe the bug: Kibana is expected to be compatible with ES+1 version out of the box to facilitate upgrade.
But Kibana 7.9.0 is not playing nice with Elasticsearch 7.10.0

Kibana 7.8.1 with Elasticsearch 7.9.0 (snapshots) work fine with each other ( I was able to add sample data, check dashboards, add filters etc)

Please note snapshots were used for testing to avoid any license differences between BC and snapshots.

Steps to reproduce:

1. Download ES 7.10.0 snapshot & Kibana 7.9.0 snapshots
2. Start ES and Kibana up
3. Kibana looks like its coming up but in browser gets stuck on space selector screen & displays "select spaces no spaces match search criteria"

Errors in browser console (if relevant):

Failed to load resource: the server responded with a status of 404 (Not Found)
:5601/33819/bundles/core/core.entry.js:84 Detected an unhandled Promise rejection.
Error: Not Found
:5601/33819/bundles/core/core.entry.js:34 Uncaught (in promise) Error: Not Found
    at Fetch._callee3$ (:5601/33819/bundles/core/core.entry.js:34)
    at l (:5601/33819/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:368)
    at Generator._invoke (:5601/33819/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:368)
    at Generator.forEach.e.<computed> [as next] (:5601/33819/bundles/kbn-ui-shared-deps/kbn-ui-shared-deps.js:368)
    at fetch_asyncGeneratorStep (:5601/33819/bundles/core/core.entry.js:34)
    at _next (:5601/33819/bundles/core/core.entry.js:34)

Provide logs and/or server output (if relevant):

Kibana logs:
logging.verbose: true

 log   [20:00:07.614] [debug][eventLog][plugins] esContext: callEs(transport.request) calls: {"method":"GET","path":"/_ilm/policy/kibana-event-log-policy"}
  log   [20:00:07.614] [debug][eventLog][plugins] writing to event log: {"index":".kibana-event-log-7.9.0","body":{"event":{"provider":"eventLog","action":"starting"},"message":"eventLog starting","@timestamp":"2020-08-13T20:00:06.461Z","ecs":{"version":"1.5.0"},"kibana":{"server_uuid":"6f5e509a-4018-467e-bc8d-38625cb9d9db"}}}
  log   [20:00:07.618] [debug][licensing][plugins] Imported license information from Elasticsearch:type: basic | status: active | expiry date: Invalid date
  log   [20:00:07.619] [info][crossClusterReplication][plugins] Your basic license does not support crossClusterReplication. Please upgrade your license.
  log   [20:00:07.623] [info][plugins][watcher] Your basic license does not support watcher. Please upgrade your license.
{ es:
   'You\'re running Kibana 7.9.0-SNAPSHOT with some different versions of Elasticsearch. Update Kibana or Elasticsearch to the same version to prevent compatibility issues: v7.10.0 @ 127.0.0.1:9200 (127.0.0.1)',
  savedObjects:
   'SavedObjects service is degraded due to Elasticsearch: [You\'re running Kibana 7.9.0-SNAPSHOT with some different versions of Elasticsearch. Update Kibana or Elasticsearch to the same version to prevent compatibility issues: v7.10.0 @ 127.0.0.1:9200 (127.0.0.1)]' }
  log   [20:00:07.625] [debug][status][plugins][spaces] unavailable: required core services are not ready
  log   [20:00:07.626] [info][kibana-monitoring][monitoring][monitoring][plugins] Starting monitoring stats collection
  log   [20:00:07.647] [debug][eventLog][plugins] esContext: callEs(transport.request) result: {"kibana-event-log-policy":{"version":1,"modified_date":"2020-08-13T18:54:42.428Z","policy":{"phases":{"hot":{"min_age":"0ms","actions":{"rollover":{"max_size":"50gb","max_age":"30d"}}},"delete":{"min_age":"90d","actions":{"delete":{"delete_searchable_snapshot":true}}}}}}}
  log   [20:00:07.648] [debug][eventLog][plugins] esContext: callEs(indices.existsTemplate) calls: {"name":".kibana-event-log-7.9.0-template"}
  log   [20:00:07.653] [debug][eventLog][plugins] esContext: callEs(indices.existsTemplate) result: true
  log   [20:00:07.653] [debug][eventLog][plugins] esContext: callEs(indices.existsAlias) calls: {"name":".kibana-event-log-7.9.0"}
  log   [20:00:08.134] [debug][eventLog][plugins] esContext: callEs(indices.existsAlias) result: true
  log   [20:00:08.134] [debug][eventLog][plugins] initializing elasticsearch resources complete
  log   [20:00:08.135] [debug][eventLog][plugins] readySignal.signal(true)

cc @legrego @LeeDr

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[bhavyarm]

Please note if I copy the data & config directory from ES 7.9.0 to ES 7.10.0 (I am doing an upgrade then) - Kibana works fine.

[LeeDr]

This appears to only be an issue in the unusual case where Elasticsearch version 7.10.0 is later than Kibana version 7.9.0 AND there's no existing .kibana index.
So this isn't the typical upgrade scenario where Elasticsearch and Kibana are running on 7.9.0 and you upgrade Elasticsearch to 7.10.0 (I just verified this works).