I think it might be caused by onBlockActivated being called when the tank is right-clicked with a spawner or the updateEntity trying to fill it. (Only if they are placed by clicking the tank - any direction, not when placing next to the tank. Only if the tank is not empty)

OpenBlocks 1.2.7 with OpenModsLib 0.4

This is bigger, vanilla bug. Probably nothing can be done, since that's how tile entities work.

So you are saying, that when i place a mobspawner against any TileEntity it will loose its Data?

So i tried some TileEntitys:
Chest, Furnace, vacuum hopper - need to hold shift - working
Fan, Rope Ladder - works without holding shift or wiping spawner data. (These are TileEntitys, aren't they?)

When holding shift while placing the spawner against the Tank also makes it keep its data.

So the fact, that you can place the spawner while sneaking or when the tank is empty makes me think its related to OpenBlocks and not a vanilla bug.

First of all, are you talking about vanilla spawners, placed with some block moving mod? If yes, then it's broken.

Placing block with TE usually consists of two phases - setting block and setting/reading TileEntity. Neighbour tank gets informed about new block (in first phase) and checks if it has tank capabilities (by checking TE). It will happen before moved TE is (re)created, so it will cause creation of new one, with default data.

Now, there is NO WAY to check if getting TE for block will cause creation of new one. So tanks are working properly.

i'm talking about vanilla spawners created with NEI or broken with force-wrench and then placed from inventory.

(Notice it only happens when not sneaking and right clicking the tank itself, but not when clicking a different adjacent block to place it)