So, you noted that schedule is as follows?

• weekly synthetic full
• monthly (classic) full
• or
• it’s also monthly synthetic full?

If you noted monthly sFull it’s due to hidden automatic policy to force sFull each 30 days, it’s in place by default, but as user won’t see it. 

It is running as the below.

 - every Thursday is synthetic full

 -1st sun and last Sunday of the month is full.

But as per the plan is configured full on every Sunday. How to find the synthetic backup configuration. 

You won’t find sFull configuration, that’s the case. It’s in place by default. Please reach support to adjust the sFull configuration based on your demands. 

With new version deployed last week, sFull is configured automatically based on the plan’s settings.

So now, if you configure weekly full based retention, the sFull will be triggered weekly. If bi-weekly, then sFull will be triggered bi-weekly, etc.