Ok, a case of RTFM :
Developing Assembly supply count is not reset when the Toner Container is replaced while printing is still possible

had the customer reset it in Adjustment/Maintenance :
In the case that the Toner Container is replaced while continuous printing is available,
warning message will not be cleared. To clear the warning message, select the
following: User Mode > Adjustment/Maintenance > Replace Toner While Printing Is Still
Possible, and then select the target color.
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