Another data point. I flew from MEX to LAS today on Volaris.
I got my COVID test yesterday in Acapulco 23 hours before my ACA-MEX flight on Aeromexico but 26 hours before my MEX-LAS flight on Volaris (separate tickets).
Got boarding passes for both flights online, no COVID test check for the ACA-MEX flight of course.
Went to gate for the MEX-LAS flight and they were making announcements to show COVID test to board and although my Spanish is not good I distinctly heard them say "24 hours."
Handed the staff my 26 hour old COVID test and no issues boarding.