I don't believe nodes have to use the "memo" field for this. Note the phrase "description hash" in this sentence: "the server should fetch a description hash invoice where the description is this zap request note and this note only"

The purpose of the description hash field is so that you can have a description larger than what fits in the memo field. Instead of putting the full description in the memo, you just hash it and put the hash in the description hash field; then you can give the full description separately to your user.

Now, given that you tested this, it's probable that you found an *implementation* bug -- specifically, you found a zap implementation that assumes the description will fit in the memo field of a lightning invoice, which might *usually* work but -- as you found -- not always. The implementation should be updated to *check* if the description will fit in the memo field. If not, it should only put a snippet in there (or nothing at all) and instead use the *description hash* field.