Rebuilding the Index through deleting files or reseeding the database does not work anymore in Exchange 2019, becaus Exchange 2019 is using BifFunnel aka Bing. The Index is part of the mailbox, so no more index files and directories.
Use get-mailboxstatistics MAILBOX | fl *bgifunnel* and check if these values are not equal 0:
The easiest way to fix an index is by moving the mailbox to another database.
Open PowerShell (not Exchange Shell) on an Exchange Server with elevated permissions and run these commands:
Start-MailboxAssistant -Identity <Mailbox id> -AssistantName BigFunnelRetryFeederTimeBasedAssistant
Take Care: The Start-MailboxAssistant Command is case sensitive!!