A good cell at stationary condition won,t get any leakage . It may be due to manufacturing defect. or it may be kept in high temp where electrolyte evaporate and increase the internal pressure of the cell and the safety vent might have got opened.

These cells (LFP-32650, kept idle for a month) were stored as new, or as used cells ?

If they were stored as "new" cells, in the 1st case, then

what are the low-high limits, extreme values, of the 1)Temperature and the 2)relative humidity[1],

in your place, during this storing time period ?

1. Many end user, commercial, electronic products and these new cells, also, have officially, different specs for some, very high, relative humidity, places.

you possible bought scrap.