The spawnblock disappearing should never be a problem as of Beta 53.1, so I recommend playing on this version (or a later/newer one) of the game if you want the best experience when using the OneBlock mod. For the drops flying away, I recommend making a platform underneath the spawnblock and hitting it from the side.

If you are still playing on Update 52, then read the old message below.

Old message (before Beta 53.1).
There are two things you can do to make the block reappear:
1. Close the world and open it again.
2. Place a block where the spawnblock is supposed to be, and break it. Then it will behave like normal again.

I sadly can’t do a check to make sure that the block isn’t air during normal gameplay, because it messes with the hit detection and makes it so that the block takes much longer to destroy (a sad side effect). So I just recommend leaving the world and joining it again. It shouldn’t disappear that often. If it does, let me know.

The blocks never actually disappear, but they sometimes fall through the block. So I recommend making a platform underneath the block.
The reason that they fly away is because you sometimes accidentally hit the block item with a tool, and therefore accidentally make them fly away. Here I also recommend that you make a platform under the block, and hit the block from the side instead. If you don’t pick up a block item when they appear on top, then when a new one appears they also push the old one away - which can also make them fly away because block items have collision :D

Under what circumstances did the block disappear? Was it totally random? Did you use a specific tool? I haven’t had it just disappear when breaking it normally - but I can make it disappear if I use two iron pickaxes at the same time and hit the block twice at the exact same time - but that’s not something I can control sadly.

Half of my blocks are flying away but I like it :D
Gives you something to think about and build something around to prevent that from happening

引用自 Volpa：

Half of my blocks are flying away but I like it :D
Gives you something to think about and build something around to prevent that from happening

The flying away part would also be quite difficult to prevent anyway haha. Seeing as block items have physics and everything, they’re pretty much destined to fly everywhere in a void world xD