Scons rebuilds everything under Ubuntu 20.04

Question

I'm doing build management for a fairly large C++ project. Under Ubuntu with Linux 18.04, using Python 2.7, scons behaves as it should - that is,

make clean
scons <command line options>
scons <command line options>

the first scons will compile all the targets, but the second scons will not compile any targets, because the timestamp dependencies will show the targets to be up-to-date. But when I do the same thing, with the same code-base, no changes at all, under Ubuntu with Linux 20.04, using Python 3.8, scons behaves differently.

make clean
scons <command line options>
scons <command line options>

BOTH scons executions re-build everything. I've tried using command line option --debug=explain, on the second scons build, and it tells me that each target is being rebuilt because ALL of its dependent files have changed. But examining their timestamps shows that they are all old. I have been studying Scons source code, but I have no idea how to debug this, and determine why it is happening.