have you enabled Multistack?

Oh ok, now I see. There is a specific option for that now. My initial issue thinking like that (without looking if there was an option for your mod at all - gets quite overcrowded by 120 mod options...) was with a specific combination on the storage setting about limiting the stack per item type combined with the stack limit.

E.g.
When setting "stack limit 0 - 15000" to e.g. 300 and setting the similar stacks to 1, it should theoretically be like 300 wood, 300 steel, 300 plasteel, 300... and so on, but it stick to 2 item types only, but when setting the stack limit back to 0 - 15000 (default), it ended up in placing all items there, which I wanted to have there, but limitless (obviously). So this confusion was letting me think that something seemed off.

The multi stacking per container/storage, imho, should be defaulted to "do allow multistacking" - activating that little check-icon next to the -/+. The majority of people use multistackable storage solutions anyways. Having that extra tickable checkbox would make things easier to comprehend and may prevent confusions like mine :D