The MariaDB server has crashed after trying to sync with the primary-active node during IST. I've uploaded 6 CSV files on the primary node with 2,3MBytes in size and after that inactive node Test5 has crashed after trying to synchronize during IST.
After loading/insert of thrird CSV file Gcacge has been created "Created page /var/lib/mysql/gcache.page.000000 of size 134217728 bytes".
CSV file is 2.3 MBytes and after the third, it is creating a new cache page with 128MB.
Gcache settings: gcache.page_size = 128M; gcache.size = 4G;

Test4 server was active, test5 was trying to sync data and it crashes after 51.4% with the message

terminate called after throwing an instance of 'std::bad_alloc'
what(): std::bad_alloc
210616 10:54:31 [ERROR] mysqld got signal 6 ;
This could be because you hit a bug. It is also possible that this binary
or one of the libraries it was linked against is corrupt, improperly built,
or misconfigured. This error can also be caused by malfunctioning hardware.

To report this bug, see https://mariadb.com/kb/en/reporting-bugs

We will try our best to scrape up some info that will hopefully help
diagnose the problem, but since we have already crashed,
something is definitely wrong and this may fail.

Server version: 10.5.10-MariaDB-log
key_buffer_size=134217728
read_buffer_size=131072
max_used_connections=0
max_threads=153
thread_count=21
It is possible that mysqld could use up to
key_buffer_size + (read_buffer_size + sort_buffer_size)*max_threads = 467871 K bytes of memory
Hope that's ok; if not, decrease some variables in the equation.

Thread pointer: 0x0
Attempting backtrace. You can use the following information to find out
where mysqld died. If you see no messages after this, something went
terribly wrong...
stack_bottom = 0x0 thread_stack 0x49000
Can't start addr2line
/usr/sbin/mariadbd(my_print_stacktrace+0x2e)[0x55cb5d02a45e]
/usr/sbin/mariadbd(handle_fatal_signal+0x485)[0x55cb5cab9b75]
/lib64/libpthread.so.0(+0x12dd0)[0x7f2634301dd0]
/lib64/libc.so.6(gsignal+0x10f)[0x7f263364970f]
/lib64/libc.so.6(abort+0x127)[0x7f2633633b25]
/lib64/libstdc++.so.6(+0x9006b)[0x7f2633de606b]
/lib64/libstdc++.so.6(+0x9650c)[0x7f2633dec50c]
/lib64/libstdc++.so.6(+0x96567)[0x7f2633dec567]
/lib64/libstdc++.so.6(__cxa_rethrow+0x4d)[0x7f2633dec81d]
/usr/lib64/galera-4/libgalera_smm.so(+0x87ab5)[0x7f26317eeab5]
/usr/lib64/galera-4/libgalera_smm.so(+0x82e69)[0x7f26317e9e69]
/usr/lib64/galera-4/libgalera_smm.so(+0x84d15)[0x7f26317ebd15]
/usr/lib64/galera-4/libgalera_smm.so(+0x8509b)[0x7f26317ec09b]
/usr/lib64/galera-4/libgalera_smm.so(+0x8513a)[0x7f26317ec13a]
/usr/lib64/galera-4/libgalera_smm.so(+0x700fa)[0x7f26317d70fa]
/usr/lib64/galera-4/libgalera_smm.so(+0x70238)[0x7f26317d7238]
/usr/lib64/galera-4/libgalera_smm.so(+0x74068)[0x7f26317db068]
/usr/lib64/galera-4/libgalera_smm.so(+0x7444b)[0x7f26317db44b]
/usr/lib64/galera-4/libgalera_smm.so(+0x94d37)[0x7f26317fbd37]
/usr/lib64/galera-4/libgalera_smm.so(+0x95f39)[0x7f26317fcf39]
/lib64/libpthread.so.0(+0x82de)[0x7f26342f72de]
/lib64/libc.so.6(clone+0x43)[0x7f263370de83]
The manual page at https://mariadb.com/kb/en/how-to-produce-a-full-stack-trace-for-mysqld/ contains
information that should help you find out what is causing the crash.
Writing a core file...
Working directory at /var/lib/mysql
Resource Limits:
Fatal signal 11 while backtracking

Also, primary node Test4 goes into non-primary from Primary