ID 351623 - Nuke needlessly re-running engine() call on mouse-over

Problem summary
Within the Nuke NDK, Nuke appears to needlessly re-run engine() calls on mouse-over.

This issue is apparent when calling _validate() and _request() calls, and then subsequently calling engine() again on an Iop in unexpected situations.

It does these unexpected requests after initiating a mouse-over the viewer. 
If there is no mouse-over the viewer,  the calls to append(), _validate(), _request() and engine() are brought in as expected. 

It also appears to be due to an Iop having a custom append() method.
It does these unexpected requests after mouse-over of the viewer.

The part that appears to cause this behaviour is calling append() on the node's inputs. 

Customer reported version
nuke.10.0v6

Customer reported platform
ubuntu

Workaround
Unknown.

Reproduced by support
This bug has been reproduced in:
( reproduced via Nuke team )

Earliest version tested
- This issue appears to be in all versions of the product

Expected behaviour
The calls should not be needlessly called on mouse-over  

Actual behaviour
Engine() calls are sent needlessly on mouse-over