Not sure if this is intended or not, but any of the below can overwrite air in fortresses:

glowstone (before 1.16 as well as now)

any type of new tree (red fungus, blue fungus)

basalt pillar (mentioned MC-171553)

Example seed: 4 (/locate fortress, search indoor section left side to find tree inside fortress)

Also fossils are able to replace solid blocks which is more concerning.

Seed: -1106935450745691990. Building portal at spawn, nearest fortress east has some overgrowth, and a different fortress nearest via /locate command has fossils replacing brick blocks.

 EDIT: Second example works only for earlier snapshot 20w16. In Pre-Rel 2 there is only one fortress as found by /locate and it has trees but no fossils due to biome changes in other snapshots.