For future readers: commit 7e8a89387b's commit message says:

 1) The ipv6 addresses were incorrectly handled in the SST script

    for rsync (incorrect address substitution for establishing a

    connection, incorrect address substitution for bind, and so on);

This issue is still present in MariaDB 10.3.30 and MariaDB 10.5.11, both of which include this commit.

This behaviour is due to wsrep_sst.cc wsrep_sst_prepare function. Variable wsrep_sst_receive_address is being used as rsync address without parsing. When wsrep_sst_receive_address is not present or is "AUTO", wsrep_node_address is used to generate the address by parsing it through wsrep_utils.h wsp::Address, which returns ipv6 address without square brackets. Square brackets are used by wsrep_sst_common.sh to determine if the address is ipv6, so all ipv6 addreses passed through wsrep_node_address will be concidered ipv4 due to lack of square brackets.

There are possible solutions:
1. Change wsrep_sst_common.sh to understand ipv6 addreses without square brackets.
2. Remove address parsing for wsrep_node_address.
3. Check if parsed wsrep_node_address is ipv6 and add square brackets accordingly.

Here is a small patch I made which both improves addres generation consistensy between methods and adds square brackets around ipv6 addreses. This is only tested to compile without errors, but I am having trouble running the tests right now, so their results will be posted later. I would appreciate if anyone else tested this. ipv6.patch

ok to push

Fixed, https://github.com/MariaDB/server/commit/1a8cf23b7b6b381aeb97f6546f52b81749155ee0