ok thx I'll make sure it only happened with Holy Proliferation + building or Ark ship + building or System dev + buildings and that it did not happen also with building + building.

Can it happen with holy proliferation+Ark ship? I don't think it can. 

Firstly because they cost essence and AFAIK you cannot rush-buy essence, and secondly by the time you reach your essence threshold, that's the exact cost of an Ark. 

Julot wrote:

ok thx I'll make sure it only happened with Holy Proliferation + building or Ark ship + building or System dev + buildings and that it did not happen also with building + building.

The simple answer is Industry cost. Everything you build has a Industry cost. If you have enough industry on a system, then you can cover the cost of all the buildings in one turn. This is why you can build multiple ships in one go as well, as they have a smaller cost than most buildings. In the case of the Vodyani special improvements, this is because they have no industry cost, hence them being able to be build instantly alongside a building. The same applies to system developments. This is very much intentional, so don't think something funny is going on

There are some buildings with fixed building times, like system updates. And there are the normal buildings which can be bought, even more of them at once. Since you can convert Industry into Dust, it's only logical to convert Dust into Industry. Dust is not only a currency, it's more like a magical ressource where the magic happens :)

Yeah, not sure why this is an issue? We humans are idiots with sticks compared to the races in Endless Space 2, and yet we manage to have multiple things built even within a city, let alone planet and/or solar system. The system is fine as is, nothing to correct.