In your Color Settings for the current working profile, trying unchecking the box "Blend Text Colors Using Gamma" and see if Merge works as expected.
In PS CC 2020, on a Mac, I was able to reproduce your issue - the lighter text got darker when I merged two text layers (I was working in sRGB). I unchecked the blend with gamma option in the working color settings and Merge works as expected.
Even when the blend with gamma option was enabled, Flatten and Stamp both worked as expected. It was just merge that caused the shift. Incidentally, if you create a 32bit per channel document and do the same experiment with merge, it works as expected.
Kirk