psql crash while executing core regression tests

hi

During installation from source code, there are many crashes for psql while executing core regression tests, all the crashes are similar, the backtrace info as follows:

Core was generated by 'psql'.
Program terminated with signal 11, Segmentation fault.
# 0 0x000000000043f140 in slash_yylex()
(gdb) bt
#0 0x000000000043f140 in slash_yylex()
#1 0x00000000004430fc in psql_scan_slash_command()
#2 0x000000000043f140 in HandleSlashCmds()
#3 0x000000000043f140 in MainLoop()
#4 0x000000000043f140 in main()

I did more compared testing about this scenario, as follows:
1. install from local source code(xxx.tar.gz)
1) switch to source tree directory, and build there  ---- no crash generated
2) create a build directory, and build there  ---- no crash generated

2. install from git source code
1) switch to source tree directory, and build there ---- no crash generated
2) create a build directory, and build there ---- many crashes generated, but if install newer version of flex, e.g. 2.6.4, the problem doesn't exist. Any suggestions about this behavior?

NOTES:
test commands are same, as follows:
configure --enable-coverage --enable-tap-tests
make
make check

testing environment:
PostgreSQL: 13.2
redhat 7.4, 3.10.0-693.e17.x86_64
flex: 2.5.37

thanks
walker