Some display values with list-item keyword can be shortened to a single keyword:

• list-item block flow can be shortened to list-item
• list-item inline flow can be shortened to inline-list-item

Some display values with list-item keyword require all three keywords:

• list-item run-in flow-root
• list-item inline flow-root

The other display values with list-item keyword can be shortened to two keywords:

• list-item run-in flow can be shortened to list-item run-in
• list-item block flow-root can be shortened to list-item flow-root

This question is about the last case. Do they serialize to the two-keyword shortened form, or do they stay in the full form?

CSS Display says

Following the precedence rules of “most backwards-compatible, then shortest”, serialization of equivalent display values uses the “Short display” column.

So it seems short forms are preferred, but these combinations do not appear in the summary table!