v4 Update/FindById fails

[Roger-Sa]

Using v4.1.4.0. Years back I created a DB where I used GUID as string as ID (for some reason), like so:

[BsonId]
public string Guid { get; set; }

It has worked like a charm for years. Now I have an item in my collection which LiteDB claims to not find:

bool updated = collection.Update(obj); // updated = false
var item = collection.FindById(obj.Guid); // item = null

But this works: var item = collection.FindAll().Where(x => x.Guid == obj.Guid);

This is not a mixup of actual GUID and GUID as string, using string all the way (it just happens to be a GUID). My question is, why might this fail? Using a string as ID should be fine I guess, right? It seems it is only this particular item, other items are found.