Repeatedly getting the error below on our 8.18.7 site it is load balanced with a single CMS box and a scaled out two instance Azure app service

Umbraco.Core.LocalTempStorage

is set to

EnvironmentTemp

Server roles are defined and umbracoApplication Url is set

Umbraco.Examine.LuceneDirectoryFactory

is set to to

Examine.LuceneEngine.Directories.SyncTempEnvDirectoryFactory, Examine

Any ideas?

System.NullReferenceException:
   at Lucene.Net.Index.SegmentReader+Norm.Clone (Lucene.Net, Version=3.0.3.0, Culture=neutral, PublicKeyToken=85089178b9ac3181: d:\Lucene.Net\FullRepo\trunk\src\core\Index\SegmentReader.cs:698)
   at Lucene.Net.Index.SegmentReader.ReopenSegment (Lucene.Net, Version=3.0.3.0, Culture=neutral, PublicKeyToken=85089178b9ac3181: d:\Lucene.Net\FullRepo\trunk\src\core\Index\SegmentReader.cs:972)
   at Lucene.Net.Index.SegmentReader.Clone (Lucene.Net, Version=3.0.3.0, Culture=neutral, PublicKeyToken=85089178b9ac3181: d:\Lucene.Net\FullRepo\trunk\src\core\Index\SegmentReader.cs:880)
   at Lucene.Net.Index.IndexWriter+ReaderPool.GetReadOnlyClone (Lucene.Net, Version=3.0.3.0, Culture=neutral, PublicKeyToken=85089178b9ac3181: d:\Lucene.Net\FullRepo\trunk\src\core\Index\IndexWriter.cs:633)
   at Lucene.Net.Index.DirectoryReader..ctor (Lucene.Net, Version=3.0.3.0, Culture=neutral, PublicKeyToken=85089178b9ac3181: d:\Lucene.Net\FullRepo\trunk\src\core\Index\DirectoryReader.cs:204)
....
PublicKeyToken=null: D:\a\1\s\src\Examine\LuceneEngine\Providers\MultiIndexSearcher.cs:82)