I encountered the same issues when using templates today as well. I was using the 10.1.0.0/16 subnetteed out per site. I had been using 10.255.0.0/16 elsewhere. So it wasn't my first template like you described. However I think regardless the issue can be fixed by what I am about to explain.
The first step is to raise this with Meraki support. You will need to request them to enable the Loose Subnets feature. What this does is prevent the Meraki logic from checking whether you are using a subnet in the same supernet as 10.0.0.0/8. Of course, you still want some sanity checking by Meraki so ask them to enable the second feature which will verify if there is overlapping subnets within the 10/8 network.
After these two features were enabled this, fixed my problem and I was able to assign my template with no issues.