Hi @Laurens,
I think atlas search by default searches as OR expression therefore to ensure that terms are included you should use the AND and Must or Should expressions.
On the other hand I don’t think single latters are being tokenized in a default analyzer. I believe you should build a custom analyzer for that
I think you should specifically look at ngrams which define the sizes of chunks.
Please note that the more granular your tokenizing will be the bigger the index might grow and this will have a performance and resources impact on your cluster.
Thanks
Pavel