The contents of the stronghold library loot table have been replaced with those of the stronghold altar loot table. Originally thought this was the chests loading with the wrong loot table, but using commands to spawn chests with the library loot table provided resources from altar instead (redstone dust, Otherside, iron armour, etc.)
Experienced with both experimental packs enabled at once.
Can confirm, it appears to only be an issue when the 1.20 experimental data pack is enabled