Ditto - garbage when console switch

We’re using ditto to monitor some installed systems.
ditto -kf /dev/con1
We have noticed that when the console switches at the target
under user control, or software control, the ditto doesn’t completely
refresh the screen to the new consoles contents.

If you use (CTRL-E then N) in ditto to switch viewing of consoles then
the screen updates OK.

But when the console switch is commanded from the target system
(CTRL-ALT-ENTER or CTRL-ALT-1…9 or in software: console_active() )
then ditto doesn’t clear any lines that are blank on the new screen, leaving
instead the text & colours from the previous console.

Anyone else seen this?
Anyone have a solution?
Currently we can use CTRL-E N CTRL-E P to refresh the console, but
proper screen control is preferred…