Chip fabrication is divided into two steps, FEOL and BEOL
The manufacturer has to start with FEOL where all the components (transistors, resistors, capacitors) are created in the silicon.
Only when FEOL is done, BEOL can start.
BEOL is the step where all the components on the chip are connected to make circuits. This is also the step which will define the mask ROM data in your chip.
Often FEOL is more complicated and takes more time than BEOL so that is why although they've started, they can still allow for changes in the BEOL part of the design.
Probably your chip vendor has split the process in two, they first make the FEOL masks and start processing. When the FEOL is almost finished they make the masks needed for BEOL. Making the BEOL masks already at the start of FEOL makes little sense as they're not needed until FEOL is finished and BEOL starts.
This also allows for ROM data updates as that is BEOL only.
So the mask needed to program the data on the chip is probably not made yet so they can easily change it even though they started making the masks already. But those will be FEOL masks, not the BEOL masks with the ROM data.