I'm using Directus lastest version 9..
So the problem is when we use Directus flow to alter the field value of any Data model, then its not getting recorded in Revision history.
Currently i'm using the "Filter" type of Directus Flow
I have raised an issue (https://github.com/directus/directus/issues/17677). Directus Team has fixed this issue and its working fine in latest directus version 9.26.