Even after RStudio upgrade, the problem resurfaced.
Although in the current manifestation it is slightly clear what is happening...and I guess someone may be able to suggest an immediate solution.
There is a red circle at the point - not a solid red dot, which it should become after sourcing the code. That looks to be the root cause of the invisible debug point.
I attach another screencast capture - now on the new version of RStudio.
This is RStudio version 1.1.447
Sticky debug point