Is including multiple data types in a field an anti-pattern?

On Venmo's web app, you create a transaction by selecting a recipient and then filling out a field that includes a dollar amount and a description:

Both the dollar amount and the description are mandatory, and they can only be entered in this field. If you just enter "8.00", for example, it's rejected--you have to use the format "[number] for [thing]".

This was confusing for me at first* since a dollar sign at the left of a field seems to indicate that it's a field for a numeric dollar amount. In fact, I don't think I've ever seen this done before, even on paper forms--all checks have separate fields for the transaction amount and the memo, for example. At the very least, this seems like bad accessibility, since some platforms may have different controls for numeric vs. text fields, and there's no good reason that I'm aware of to require users to enter a monetary amount as text.

Are there any valid reasons to create fields like this?

*On Venmo in particular, this is exacerbated by the unhelpful error message A transaction must have a note, e.g., "Kale salad with beans, onions, mushrooms, berries, and seeds.", which goes into details about someone's salad but fails to provide an example of correct input. But I'm wondering about this field format in general.