I recently upgraded my macOS GNAT from 2015 to Simon's latest offering ("if it ain't broke...") on new Apple hardware. At the same time I used Apple's Migration Assistant to transfer files. From Visual Studio Code, I use a short shell script to run gprbuild. This has worked flawlessly for quite a while. Now things are broken and I can't figure out what is wrong even after reviewing the gprbuild docs.

GPRBUILD 23.0.0 (20221109) (aarch64-apple-darwin21)

Here is the shell line invoking gprbuild
gprbuild -p /Users/me/Documents/Programs/Ada/Code/My_Projects/TextMate_Sampling/build.gpr

Here is the gpr file named build.gpr after deleting a bunch of comments (sorry for the length);
with "MPFR_Library"; -- Multiple precision floating point ops and functions
with "GMP_Library"; -- Multiple precision integer ops and functions
with "Octave_Library"; -- Octave built-in functions (not the loadable functions from Octave)
with "GSL_Library"; -- GNU Scientific Library, installed as part of Mac Ports.

project Build is
for Source_Dirs use
("/Users/me/Documents/Programs/Ada/Code/My_Projects/TextMate_Sampling/source/",
"/Users/me/Documents/Programs/Ada/Code/",
"/Users/me/Documents/Programs/Ada/Code/My_Projects/TextMate_Sampling/source/Tests",
"/Users/me/Documents/Programs/Ada/Code/My_Projects/TextMate_Sampling/source/Other_Code",
"/Users/me/Documents/Programs/Ada/Code/My_Code/Specialized/Sampling",
"/Users/me/Documents/Programs/Ada/Code/My_Code/Specialized/Radar",
"/Users/me/Documents/Programs/Ada/Code/My_Code/Specialized/Sampling/Post-processing",
"/Users/me/Documents/Programs/Ada/Code/My_Code/My_Tools",
"/Users/me/Documents/Programs/Ada/Code/My_Code/One-Offs",
"/Users/me/Documents/Programs/Ada/Code/My_Code/Examples_and_Snippets_and_Notes",
"/Users/me/Documents/Programs/Ada/Code/Bindings/PLplot/plplot_git/plplot/bindings/ada",
"/Users/me/Documents/Programs/Ada/Code/Bindings/PLplot/plplot_git/plplot/examples/ada",
"/Users/me/Documents/Programs/Ada/Code/Bindings/PLplot/PLplot_testing/",
"/Users/me/Documents/Programs/Ada/Code/Bindings/GMP_and_MPFR/SVN/adabindinggmpmpfr-read-only",
"/Users/me/Documents/Programs/Ada/Code/Bindings/Octave/",
"/Users/me/Documents/Programs/Ada/Code/Bindings/GSL/");

for Object_Dir use "build-normal";
for Exec_Dir use "product-normal";
for Main use ("Uniform_Aperture_Sum.adb"); -- <<< SET MAIN HERE

package Builder is
for Default_Switches ("Ada") use ("-O0", "-gnat12", "-g", "-gnatE", "-gnateE");
for Executable ("Uniform_Aperture_Sum.adb") use "run"; -- <<< AND HERE
end Builder;

package Compiler is
end Compiler;
package Binder is
for Default_Switches ("Ada") use ("-E");
end Binder;

package Linker is
for Default_Switches ("Ada") use ("-Wl,-no_pie");
end Linker;

package Pretty_Printer is
for Default_Switches ("ada") use ("--max-line-length=100", "--indentation=4",
"--indent-continuation=2", "--decimal-grouping=3", "--based-grouping=4", "--alignment",
"--RM-style-spacing", "--preserve-blank-lines", "--comments-fill", "--comments-special",
"--no-separate-return", "--split-line-before-op");
end Pretty_Printer;

end Build;

Here is the path to the main program. The path is in the sources list in build.gpr.
/Users/me/Documents/Programs/Ada/Code/My_Code/Specialized/Radar/Uniform_Aperture_Sum.adb

And here is the complaint from GNAT when I try to build:
build.gpr:61:19: "Uniform_Aperture_Sum.adb" is not a source of project "build"
gprbuild: problems with main sources

Both Simon's suggestion
export GNAT_FILE_NAME_CASE_SENSITIVE=0

and Egil's suggestion
package Naming is
for Casing use "MixedCase";
end Naming;

solve the problem.

Thanks. Not sure what the underlying problems are.

Jerry