The manufacturer has suggested the following solutions to the problem:

The negative pressure occurs when the air in the cup cannot flow in while the child is drinking. This can cause the straw to be compressed and make drinking more difficult. If the negative pressure becomes too strong, we recommend cleaning and refitting the straw cup:

- Disassemble the cup and clean all parts.
- Make sure that the lid is properly screwed onto the cup and that the straw is firmly seated in the opening.
- Screw the lid onto the bottle. Note: Do not screw in too tightly as this could cause liquid to leak out.
- If the vacuum becomes too strong, open the lid briefly to equalise the air.

It was the same for us. A very stupid cup in my opinion...