suddenly not threading sent messages

[Kate Nepveu]

Literally between one day and the next, emails sent by this script stopped threading / showing up in conversation view with the messages they are replies to

* Emails sent Thu Mar 31 2016 00:56 -- thread properly. 

* Emails sent Fri Apr 01 2016 00:56 -- showing up as single messages, outside conversation view.

Is anyone else experiencing this? Anyone find a workaround?

(I thought it might have to do with scheduling replies to other than the last email in a thread, but I just tested that and it doesn't.)

Thanks!