Ae Template Re-sizes Spontaneously in PPr...

Hi Guys

I'm working in PPr 2019 and I've a 10 second template (made in Ae CC2018), which I've used a 100 or so times in CC2018 and about 20 times in 2019, with no problems. I've made an 1080x1080 sequence for Instagram and exporting 'match source. medium bitrate', like I have done dozens of times. The sequence plays fine in the program monitor and renders fine, but today as soon as the play head hits the template, it shifts off centre until a second before the end. Here's a capture:20181229 AdobeAeTemplate - YouTube.

Can anyone throw any light on this? A thirty second promo is taking me all day...

Regards,

Graham