So I have tried this with both Dynamic text fields and drop down list fields.

If you are using maxdropdown... to get a look ahead field, and if you have it set to multiple values, and you delete the first value so that the second value (which was blank because you had just started typing the first one) becomes the first value, the drop down stops working. If you go into the second value that one works fine, just that very first one will not work. 

You can fix it by just using that second one and then deleting what was the first, but its going to drive users crazy. I have tried this on multiple machines. Our drop down list is around 5000 items, but the performance is fine when it works.

We are using 9.1.1

Can anyone else comment on if they have seen this or can recreate it.

Thanks