The cap is part of the intended reservoir arrangement. A loose, damaged or incorrectly seated cap can affect conditions inside the dishwasher and needs attention.
Understanding this situation
Check the manual's refitting method without cross-threading or forcing it. Grains or debris near the seating area should be handled only through the permitted cleaning process.
A useful next step
Do not continue normal use with a damaged cap or improvise a seal with tape. Identify the correct part using the complete model. Report unusual deposits or taste concerns alongside the cap issue rather than assuming every mark is caused by detergent.
Manuals and supporting guidance
Model-specific instructions take priority. These references support the practical principles discussed here; they do not diagnose your appliance.