Hi Partner,
Actually it is by design, whatever in workflow or power automate, "To" field is unavailable.
And since you want all the emails related to the contact, you could check the emails in the timeline, "From","To" and "Regarding" are all shown in the timeline list.
There is another way, you could use plug-in to check if the "To" value is contact, and if it is, then copy the value to a custom contact lookup field on the email form, and create a new subgrid to show the emails related by the custom contact lookup.
Regards,
Leo