Steps to Reproduce
I’ve attached a Zip file with a Plugin (used with Stock UE) for everything you need to repro the issue.
When in editor open BP_TOptionalBase and BP_TOptionalDerived
In BP_TOptionalBase make an edit to Raw Property and to Optional Property
In BP_TOptionalDerived see that the Raw Property change propagates whereas the Optional Property change doesn’t
(This is without any changes to the Derived BP so it’s not Instance overriding Default)
Plugin is made in 5.5.4 but same problem is experienced in 5.6.1