Up/Downconverter issues
The FS1000A module you are using as a local oscillator is nominally 433.92 MHz +/- 75 kHz and this can mean that after up-converting the frequency is 75 kHz higher than you thought.

Now, when it comes to down converting the tolerance could work with you and you get precisely your 125 kHz signal from the down-converter but that would be really, really naive.

Whether this is the problem you are getting or not this IS a design problem.

If the net frequency difference between the two FS1000A modules is 20 kHz then your down-converted signal could be 105 kHz or 145 kHz.