There are two possible scenarios for the issue:
- If one of the indexes is not accessible on Sitecore XP startup, the instance do not try to reinitialize it.
- If the Solr server is down on XP startup, Sitecore XP does not try to reinitialize the indexes that use the SwitchOnRebuild feature in the SolrCloud mode.
To resolve the issue, download and install the patch available on this page:
https://github.com/SitecoreSupport/Sitecore.Support.163850.171950/releases.
To get a solution for other Sitecote XP releases contact Sitecore Support.