Using latest pandoc-nightly-windows-2025-05-20.

The table

+----------+----+
| h1       | h2 |
+:===:+===:+:===+
| A        | B  |
+-----+----+----+
| C   | D  | E  |
+-----+----+----+

will give the following correct alignments in the AST when read:

    [ ( AlignCenter , ColWidth 8.333333333333333e-2 )
    , ( AlignRight , ColWidth 6.944444444444445e-2 )
    , ( AlignLeft , ColWidth 6.944444444444445e-2 )
    ]

However, when written as a grid_table, we get:

+-------------+----+
| h1          | h2 |
+:===========:+:===+
| A           | B  |
+------+------+----+
| C    | D    | E  |
+------+------+----+

So we lost the right-alignment on the second column (which should affect cell D for example).
The expected result should look like the original table.

For the table:

+-----+----+----+
| h1  | h2 | h3 |
+:===:+===:+:===+
| A        | B  |
+-----+----+----+
| C   | D  | E  |
+-----+----+----+

we get:

+------+------+----+
| h1   | h2   | h3 |
+:=====+=====:+:===+
| A           | B  |
+------+------+----+
| C    | D    | E  |
+------+------+----+

which is also problematic: the first column is now left-align.

For the table:

+----------+----+
| h1       | h2 |
+:===:+===:+:===+
| A   | F  | B  |
+-----+----+----+
| C   | D  | E  |
+-----+----+----+

we get a proper result.

So it's the influence of the cell colspan below which cause trouble, at least in these examples.

The alignments being a global property of the table, they should not be affected by nearby cell colspans.

A suggestion would be to always write all the alignements segments on the header separator/border.
As a compromise, one could relax this when there is no ambiguïty (eg for default aligns on columns affected by nearby colspans).