Email Template field not populated when the email template is generated

Hello,

I am created an action button which will open a pre-composed email using an email template.  All very normal stuff.
(note, Not 'process email template', just the 'email template')

I have a custom field in our Leads module which is an auto increment and lets us give each Lead a number.

This is all fine... and has been in place for years.

   

I now want to add this field into the email template, but it never populates iwhen the email is generated?
Other fields do, but not this one.

  

In the email template Insert Variable section I can find the field: (i think this is the field anyway!)

So that comes up as:  $contact_lead_number   

  • This just results in a blank where is should be in the template after clicking the button:
      

    Results in:


It is an existing record too, not a new record, so the DB definitely has a value in there:
If I use Postman with endpoint (GET): {{url}}Leads/_record ID here_  (to get everything for that record)

I can confirm the field is the correct one and has a value: 
"lead_number_c": 4943

  

In the email template I have tried using  just $lead_number_c, but that just prints into the email as written...
So does trying $lead_lead_number_c  (thinking instead of contact_lead_number_c)

  

I must be missing something?  Or does it sound a bit buggy?

Thanks in advance for any tips,
Luke.