Tested on

• Foundry 13.351
• DnD 5e v5.2.4 (set on the 2024 ruleset)
• Module activated : none bar the PHB / MM / DMG

When creating spellcast activities on NPCs, if they use a PHB spells, that also exists in the SRD 5.2, the whole feature disapear when the PHB is de-activated instead of using the SRD 5.2 spell, as shown in the following screenshots.

• Sample actor with a single feature, itself having one spellcast activity for the fly spell of the PHB

• Same actor, with the PHB deactivated

• There is an error in the console when the NPC's character sheet with the feature is opened

Note :

• I've tested summons and the monster of the MM are replaced by the equivalent in the SRD 5.2 if the MM is deactivated.
• The spell links in journals made with PHB spells will link to the SRD 5.2 ones if the PHB is deactivated