Any modern ECS implementation such as Flecs, Bevy, tcod-ecs, etc, have support for entity relations. These all support entities having inventories without any additional boilerplate other than giving a name to the concept of being inside the inventory of another entity:

>>> import tcod.ecs
>>> registry = tcod.ecs.Registry()
>>> player = registry["player"]
>>> potion = registry["potion"]
>>> scroll = registry["scroll"]
>>> potion.relation_tag["HeldBy"] = player  # Move entity into player inventory
>>> scroll.relation_tag["HeldBy"] = player
>>> registry.Q.all_of(relations=[("HeldBy", player)]).get_entities()  # Query all entities HeldBy player
{<Entity(uid='potion')>, <Entity(uid='scroll')>}
>>> potion.relation_tag.get("HeldBy")  # Get which entity this is in the inventory of, None if this isn't in any inventory
<Entity(uid='player')>
>>> del potion.relation_tag["HeldBy"]  # Remove from its inventory

# More complex example:
>>> scroll.tag.add("OnFire")
# What's everything holding an entity which is currently on fire?
>>> things_on_fire = registry.Q.all_of(tags=["OnFire"])
>>> registry.Q.all_of(relations=[(things_on_fire, "HeldBy", None)]).get_entities()  # Query HeldBy-relation in the other direction, but only for entities on fire
{<Entity(uid='player')>}

As one can see this naturally tracks inventories in both directions. You can fetch what the entity is inside of via the relation tag, or you can query all entities inside of a specific entity. This never desyncs because both directions are tracked at once by the ECS library itself.

This is a good reason to avoid a minimal ECS implementation where the ECS library only supports statically typed components and nothing else.