R12/R13 is loaded only once per frame, in MA and MA', at the first line of the first character of the frame. The counter MA then reload MA' at each raster line start. And at each new character line start, MA' captures the current value of MA.
The exception is the CRTC 1 for which the MA is reloaded at each line start with R12/R13 instead of MA' as long as VCC=0.
This is a major source of incompatibility if the programmer does not take care of this discrepancy. In demos and games, to make a display compatible with all CRTCs, program R12/R13 when VCC!=0. This will then take effect at the next frame start.