sp_Blitzcache doesn't return as many results as sys.dm_exec_query_stats for a given query
Yes, there's a known issue with that parameter not returning all results due to spaces, and I wouldn't be surprised if that's also affecting you:

https://github.com/BrentOzarULTD/SQL-Server-First-Responder-Kit/issues/2202

However, let's zoom out a little: you can't rely on a text search of the plan cache to find everything updating a table. You could have multiple spaces, tabs, even line returns or comments between the word UPDATE and the start of your table name. The table name might also be prefixed with the database name or schema name.

As a result, I wouldn't recommend this technique to find queries that update a particular table.