Sitecore XDB reindex not progressing past 95%

My team got this problem on a 10.2 XP production instance, after Sitecore Managed Cloud changed us over to their new Solr cluster.

Stage: Finishing; Stage progress: 0.00%; Total progress: 95.00%
Stage: RebuildingIncomingChanges; Stage progress: 0.00%; Total progress: 85.00%.

It then stays like this indefinitely.
Logs are showing:

2025-07-18 05:40:47.361 +00:00 [Error] Failed indexing next set of changes. There will be an attempt to recover from the failure.
Sitecore.Xdb.Collection.Data.SqlServer.Exceptions.SqlDataProviderException: *** [xdb_collection.GetContactsChanges], Line 27. Errno 50000: Sync token is no longer valid for [Contacts] table. ---> System.Data.SqlClient.SqlException: *** [xdb_collection.GetContactsChanges], Line 27. Errno 50000: Sync token is no longer valid for [Contacts] table.
at System.Data.SqlClient.SqlCommand.<>c.<ExecuteDbDataReaderAsync>b__180_0(Task`1 result)
at System.Threading.Tasks.ContinuationResultTaskFromResultTask`2.InnerInvoke()
at System.Threading.Tasks.Task.Execute()

We tried to rebuild multiple times but always it would say at 95%.

After back and forth with Sitecore support for 3 months, they finally figured out the fix: change MaximumCommitMilliseconds setting to 1000 in the \App_data\jobs\continuous\IndexWorker\App_data\Config\Sitecore\SearchIndexer\sc.Xdb.Collection.IndexWriter.SOLR.xml_file._

Happy Sitecoreing!