ok, so I was having a little trouble tracking it down because after reverting I had to remove then re-apply the customization to reproduce the problem.
I think it is actually the hbr commit @markvanlan (DEV: Use .hbr for raw template file extension (#8883) · discourse/discourse@3e89774 · GitHub), sorry for waffling on that.
To test I added this to the current theme’s </head> via the customize section on the admin panel:
<script type='text/x-handlebars' data-template-name='mobile/list/topic-list-item.raw'>
test
</script>
On the commit before this works fine, once I apply that commit and then re-apply the customization the template is ignored.