I've found two mined txs with no fee I was debugging my little project and I've found this pair of txs without fees (they caused an alarm):

[txId=e8f6f08430ab6b1f3e645f499cf90c69acf2eb8a8d9766abb9d76314f29209dd, fees=0, weight=879600], [txId=cf7ba67c57d72dc735c200f5bf81789f6464adf65816b7640562d38a8c706a05, fees=0, weight=16052]}

both in block 618858.

first one is very heavy (almost a quarter of a block). 

just curiosity. Do I have to suppose that it's probably the miner the one who has auto-sent this tx?
A transaction having a fee is not a consensus rule. Although the vast majority of nodes refuse to relay a 0 fee transaction, it is valid.

Such transactions can be included in a block by directly passing them to a miner, and would tend to belong to the miner themselves, or someone they have some kind of relation with.