PostGIS ST_Covers doesn't match polygon after ST_Union

I have a simple table:

create table test (
    id serial,
    polygon geometry(Polygon, 3857)
);

And I want to insert two polygons in this table which should be created as points expand with intersection area between them:

insert into test(polygon)
values(ST_Transform(ST_Expand(ST_Transform(ST_SetSRID(ST_MakePoint(30.2833756, 50.4419441), 4326), 31997), 100), 3857));
insert into test(polygon)
values(ST_Transform(ST_Expand(ST_Transform(ST_SetSRID(ST_MakePoint(30.2841370, 50.4419441), 4326), 31997), 100), 3857));

After that I have a query to check if point is covered by polygons union:

with point_polygon as (
    select ST_Transform(ST_Expand(ST_Transform(ST_SetSRID(ST_MakePoint(30.2841370, 50.4419441), 4326), 31997), 100), 3857)
)
select ST_Covers(ST_Union(polygon), (select * from point_polygon))
from test
where ST_Intersects(polygon, (select * from point_polygon));

I can’t understand why above query returns true for the first point and false for the second…

If I decrease the expand radius in select from 100 to 99.999 it returns true:

with point_polygon as (
        select ST_Transform(ST_Expand(ST_Transform(ST_SetSRID(ST_MakePoint(30.2841370, 50.4419441), 4326), 31997), 99.999), 3857)
    )
    select ST_Covers(ST_Union(polygon), (select * from point_polygon))
    from test
    where ST_Intersects(polygon, (select * from point_polygon));

Is it a rounding issue?