Postgres automatically switch to READ-ONLY mode - ERROR: cannot execute UPDATE in a read-only transaction - postgresql

Question

When I increase load on PostgreSQL server, by hitting REST APIs through LOAD Testing Tool. Postgres starts throwing exception ERROR: cannot execute UPDATE in a read-only transaction. Once it starts throwing exception, Even If I try to execute a update query directly to PG SQL command line, same exception is thrown there also. Then around 5-10 minutes later, it automatically comes in normal mode.
In the test suite, I have multiple APIs performing mulitple select, update, insert operations on various tables. Here We have one such table having a column with bytea data type stores a excel file in binary format. Don't know this information is relevant or not.
But my concern is why Postgres automatically switches to READ-ONLY mode, that causes my APIs fails abruptly. It is an intermittent issue, we need to increase lots of load on database server to replicate it.I am not sure it is happening due to LOAD or above mentioned "bytea" column having large binary data causing such issue?
PostgreSql version: 12
PG Driver: 42.2.18
Any suggestion or clue please?
below are logs:
Caused by: org.hibernate.exception.GenericJDBCException: could not execute statement
at org.hibernate.exception.internal.StandardSQLExceptionConverter.convert(StandardSQLExceptionConverter.java:54) ~[hibernate-core-4.3.5.Final.jar:4.3.5.Final]
at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:126) ~[hibernate-core-4.3.5.Final.jar:4.3.5.Final]
at org.hibernate.engine.jdbc.spi.SqlExceptionHelper.convert(SqlExceptionHelper.java:112) ~[hibernate-core-4.3.5.Final.jar:4.3.5.Final]
at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:190) ~[hibernate-core-4.3.5.Final.jar:4.3.5.Final]
at org.hibernate.engine.jdbc.batch.internal.NonBatchingBatch.addToBatch(NonBatchingBatch.java:62) ~[hibernate-core-4.3.5.Final.jar:4.3.5.Final]
at org.hibernate.persister.entity.AbstractEntityPersister.update(AbstractEntityPersister.java:3281) ~[hibernate-core-4.3.5.Final.jar:4.3.5.Final]
at org.hibernate.persister.entity.AbstractEntityPersister.updateOrInsert(AbstractEntityPersister.java:3183) ~[hibernate-core-4.3.5.Final.jar:4.3.5.Final]
......
....
Caused by: org.postgresql.util.PSQLException: ERROR: cannot execute UPDATE in a read-only transaction
at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2553) ~[postgresql-42.2.18.jar:42.2.18]
at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:2285) ~[postgresql-42.2.18.jar:42.2.18]
at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:323) ~[postgresql-42.2.18.jar:42.2.18]
at org.postgresql.jdbc.PgStatement.executeInternal(PgStatement.java:473) ~[postgresql-42.2.18.jar:42.2.18]
at org.postgresql.jdbc.PgStatement.execute(PgStatement.java:393) ~[postgresql-42.2.18.jar:42.2.18]
at org.postgresql.jdbc.PgPreparedStatement.executeWithFlags(PgPreparedStatement.java:164) ~[postgresql-42.2.18.jar:42.2.18]
at org.postgresql.jdbc.PgPreparedStatement.executeUpdate(PgPreparedStatement.java:130) ~[postgresql-42.2.18.jar:42.2.18]
at com.zaxxer.hikari.pool.ProxyPreparedStatement.executeUpdate(ProxyPreparedStatement.java:61) ~[HikariCP-2.4.5.jar:na]
at com.zaxxer.hikari.pool.HikariProxyPreparedStatement.executeUpdate(HikariProxyPreparedStatement.java) ~[HikariCP-2.4.5.jar:na]
at org.hibernate.engine.jdbc.internal.ResultSetReturnImpl.executeUpdate(ResultSetReturnImpl.java:187) ~[hibernate-core-4.3.5.Final.jar:4.3.5.Final]
... 46 common frames omitted