I am having the same issue with v2.4.1. I map the fields and hit OK when everything looks good. You can actually see the fields you have mapped turn red again while the tool is closing. Open it back up and everything has reset. Very frustrating, especially when you have a lot of intricate fields to map with multiple inputs and merging.
After some experimenting, if after you map a field, you sort of click around the dialog box, near where you put in the expression, it shows a sort of loading graphic in the top left. After doing this it actually saved for me but not reliably.
I also have another related issue of have multiples of the same field mapped when revisiting a tool. In the image, I have only 1 table that should have 1 field mapped to each, yet 3 are showing up for many of them.