If you have an input on the multiviewer, it correctly displays a tally frame if it is cut on-air. However, if you put that input into an overlay and toggle it on (with a different input on-air), it won't light up with a tally frame on the multiviewer.

The multiviewer should reflect the tally state of an input correctly: If it is on-air directly, or on-air via an Overlay, it should display a tally frame on both occurrences.

This is similar to how a physical vision switcher would operate, whereby any source would contain tally information if it was on-air through any method (on the program bus, or as a DSK, or keyed within a M/E row etc)