NOT_ENOUGH_SPACE in NVS after writing some log (NVS) entries in another namespace

[megawarp]

Hi,
I save in the NVS a "big" (1 - 2 Kbytes) System Structure (a struct saved as a blob) which every now and then is modified and written again in the NVS. Meanwhile I have many small LOG entries (again: struct, blobs) handled like a circular buffer, that are written and rewritten in the NVS. It happens that when I write exactly 25 log entries then when I try to update the System Structure the nvs_set_blob returns an ESP_ERR_NVS_NOT_ENOUGH_SPACE error. Then I have to erase the flash and reprogram the firmware to return to the initial state.
If I increase the size of the System Structure the problem happens after 3 log entries instead of 25.
The System Structure and the logs are in different namespaces of the same NVS partition. The NVS partition is only one (the default one) and nvs_get_stats reports that there are more than 100 free entries yet. Both the system structure and the log structure are fixed-sized.
Why do I get that error? Where is the problem?