BUG #14450: inconsistent results on async slave

The following bug has been logged on the website:

Bug reference: 14450
Logged by: Rikard Pavelic
Email address: rikard(at)ngs(dot)hr
PostgreSQL version: 9.3.14
Operating system: Linux
Description:

Before applying new version of the function to the database I wanted to
check if results of the function were the same

So we ran a query like this:

select a.id, original_fun(a.id), new_fun(a.id)
from table a
where coalesce(original_fun(a.id), MIN_DATE) != coalesce(new_fun(a.id),
MIN_DATE)

on all our async slave instances.
Strangely for small number of rows (6 out of 500k) this query returned
different results.
After running:

select original_fun(diff_id), new_fun(diff_id);

for one of those ids, the function returned the same result.

Is this expected behavior?

original_fun = plpgsql (volatile - should be stable)
new_fun = sql

What is even more strange, is that different results were like this:

id1, X1, null
id2, null, X2
id3, X3, null
id4, X4, null
id5, X5, null
id6, X6, null

data is only inserted and X_ represents min date of sum subset of data in
other tables.

Regards,
Rikard