UPDATE: Same solution worked from upgrading 20.0.5 to 21.0.2.
Config your quarkus.porperties with this variable:
quarkus.transaction-manager.default-transaction-timeout=35M
Value can be adjusted, based on the size of the realms. 35M worked seamless with +400 realms. As well as more than +600, see: Align `quarkus.transaction-manager.default-transaction-timeout` with storage lock timeouts · Issue #19453 · keycloak/keycloak · GitHub