Steps to reproduce

Use the following wikicode to create a table (or look at my sandbox page where I have done it for you).

{| class="wikitable sortable mw-collapsible mw-collapsed"
|-
! colspan="3" |Big header
|-
! Column 1
! Column 2
! Column 3
|-
|-
| Value 1
| Value 2
| Value 3
|}

Run the same code again, but then without the sortable or mw-collapsed tags.

Actual result

When the table is expanded, the big header and the three values are visible, but the column names aren't.

When the mw-collapsed class is not present, the column names are always visible, just like the big header.
When the sortable class is not present, the column names appear if and only if the table is expanded.

Expected result

When the table is expanded, all header rows should be visible.