Comment 8 for bug 1926355

OK, I've tested it in clean multipass VMs and test-snapd-rsync-core20.rsync does not work on Bionic, Focal and later. Interestingly it ships a local copy of libc6 inside that could be the problem and it worked on Focal for some time due to accidentally matching the host's libc6.

...
Reading target:/usr/lib/debug/snap/test-snapd-rsync-core20/11/lib/x86_64-linux-gnu//libc-2.31.so from remote target...

Program received signal SIGSEGV, Segmentation fault.
0x000055eb84b7b2d4 in time@plt ()
(gdb) info sharedlibrary
From To Syms Read Shared Object Library
0x00007f070255d100 0x00007f070257f7c4 Yes (*) target:/lib64/ld-linux-x86-64.so.2
                                        No linux-vdso.so.1
0x00007f07025514f0 0x00007f07025557e8 Yes (*) target:/snap/test-snapd-rsync-core20/11/usr/lib/x86_64-linux-gnu/libacl.so.1
0x00007f0702543720 0x00007f070254a92d Yes (*) target:/snap/test-snapd-rsync-core20/11/usr/lib/x86_64-linux-gnu/libpopt.so.0
0x00007f0702374630 0x00007f07024e908f Yes (*) target:/snap/test-snapd-rsync-core20/11/lib/x86_64-linux-gnu/libc.so.6
(*): Shared library is missing debugging information.

For the record test-snapd-rsync-core18.rsync does not ship and internal libc copy and does work on all releases I tried (Bionic, Focal).