pg_visible_in_snapshot clarification

   I'm looking for some clarification around pg_visible_in_snapshot function.  As it seems to not be working the way I would expect or the way the documentation is setup.

I've attempted this on pg13 and pg15 and am getting the same behaviour.

3 connections via psql 2 to the primary and 1 to the secondary

C1: primary

C2: primary

C3: secondary (setup using streaming replication with hot_standby_feedback on)

Reproduction:

C1:

CREATE TABLE test (

id integer,

val text);

BEGIN TRANSACTION;

INSERT INTO test values (1, 'test 1');

SELECT pg_current_xact_id();

C3:

SELECT pg_visible_in_snapshot('<xact id from select above>'::xid8,pg_current_snapshot()); -- returns false

SELECT pg_current_snapshot(); -- shows <xid from C1>:<xid from C1>:

C2

BEGIN TRANSACTION;

C3

This is where things don't make sense to me.  I would expect

SELECT pg_visible_in_snapshot('<xact id from C1 select above>'::xid8,pg_current_snapshot()); -- returns true

to return false as the transaction on C1 is still open and not commited.  The 1,test 1 record in the test table is not available on the secondary yet however 

pg_visible_in_snapshot is returning true for it's xactid. 

I think this has to do with the pg_current_snapshot not showing the transaction in the xip_list which appears to be empty on both C1 (in the transaction) and C3 on the replica.  However C2 pg_current_snapshot() does show C1 xactid as in progress in the xip_list.

So I'm guessing from this that pg_visible_in_snapshot is not safe to use between a primary and a secondary?  

If anyone could provide any additional insight that would be amazing.

Thanks