Postgresql – Max does not return maximum timestamp value

In PostgreSQL 16.1 I have a very large table mytable with a column _modified with type Timestamp with time zone. When I run below query on the whole table, it returns a lower timestamp value compared to when I run the query on as subset of the table.

A query on the whole table returns 06-11-2023 17:06:38.000:

select max(cast(_modified at time zone 'UTC' as timestamp))
from mytable 
;

A subset of the table returns 11-06-2023 17:06:38.001:

select max(cast(_modified at time zone 'UTC' as timestamp))
from mytable 
where mycondition = true
;

I would expect the query without where clause to return at least the value of a subset of the query. Not sure if it is related but I run these queries in DBeaver. Anyone any idea how this could happen and how to force the max operator to return the correct value?