I don't if you followed through on this but I've had the same problem several times.
It seems this happens everytime you select a time period that is outside of the model calendar but is inside one of your time ranges. In the classic UI everythings works fine but in the New UX, it displays just "Find...". It does not have any impact on calculations, it is just a display problem.
What is the calendar setting of the module in which the line items sits? If you are using a time range just for the line item while the overall module time frame is set as the calendar time this may be what is causing your problem. In the UX you allocate the module to the field card and I suspect that the module settings are then applied universally to that card.