I received a reply from ESA support about this:
Indeed, a change occurs with Processing Baseline 04.00 in January 2022 with the introduction of a radiometric offset allowing to keep these negative surface reflectance values.
It appears that this change has not been handled correctly for the generation of the TCI in the standalone version of Sen2Cor. The Sen2Cor team has reproduced the issue observed by the user, and a fix will be proposed soon in a future version of Sen2Cor.
On the other hand, the issue does not appear in the official ESA products as the TCI is generated in another way.
To conclude, it is true to say that the reflectances in the TCI are coded between 1 and 255, 0 being reserved for ‘No Data’. So far, using the standalone version of Sen2Cor, a 0 in the TCI could also depict a negative reflectance in the B02, B03 or B04 band. But this is a wrong behaviour that will be fix soon.