Instant replay 120p/60p cannot be mixed

There are network cameras with 120p output.
In order to use it for instant replay in sports broadcasting,
Must be able to mix naturally with 60p sources

Instant replay of vmix can be mixed with 30/60p sources.
It is not possible to mix 60p and 120p.

Condition :
fhd 120p project

input: 120p rtsp, 60p hdmi
instant replay setting: fhd 120p, bitrate low

Problem :
Frames in 60p should be doubled to fit 120p, but it doesn't