I tried the notebook with Catalina only. The behaviour is the same in both 12.1 and 12.0 for me -- I had to remove and restore the colon to make it "compile".
So I can't help with the problem but I have some observations to share:
I run "diff" on the original and "corrected" version.
Original version contains:
RowBox[{"nPoints_", ",", "setUpAltsFunc_", ",", " ",
RowBox[{RowBox[{"radius_", ":"}], "1.0"}], ",", " ",
RowBox[{RowBox[{"scaleFac_", ":"}], "0.0014"}]}]
The same part in modified version:
RowBox[{"nPoints_", ",", "setUpAltsFunc_", ",", " ",
RowBox[{"radius_", ":", "1.0"}], ",", " ",
RowBox[{"scaleFac_", ":", "0.0014"}]}]
You can see that in original version splits pattern between RowBox differently -- in original version defaults are separated from the pattern.
