Wrong PERCENT_RANK computation in Firebird 5

[lukaseder]

Run this query:

select
  a,
  percent_rank() over (order by a desc) "p1"
from (
  select 1 a from rdb$database union all
  select 2 a from rdb$database union all
  select 3 a from rdb$database union all
  select 4 a from rdb$database
) t
order by a asc

The correct result as produced by Firebird 4 was:

1	1
2	0.66666666666667
3	0.33333333333333
4	0

Firebird 5.0.3 now produces:

1	0.1111111111111111
2	0.16666666666666666
3	0.3333333333333333
4	0.0

I'm using Firebird from here: https://hub.docker.com/r/firebirdsql/firebird.

A workaround, I can calculate percent_rank() over (order by a desc) myself using rank() over (order by a desc) - 1.0) / (count(*) over () - 1.0)