Currently Flow supports record leve triggers for Dynamics 365 customer engagement. To make Flow more powerfull and we should be able to trigger on a field change within Dynamics.
Given that Flow is consumption based, its doesn't seem very cost effective to have a flow run on record change especially you have a Flow run on high volume records such as Contacts or Accounts.
Appreciate you could use a work around utilising the Dynamics workflow and a custom entity but its not a great experaince