When I launch my server running 1.14.2 pre-2 with the "--forceUpgrade" tag I get an error spam that looks like this:

[pool-3-thread-1/WARN]: Unable to resolve BlockEntity for ItemStack: minecraft:spawner
[pool-3-thread-1/WARN]: Unable to resolve BlockEntity for ItemStack: minecraft:air
[pool-3-thread-1/WARN]: Unable to resolve BlockEntity for ItemStack: minecraft:air
etc.

The picture shows what it looks like.

Not sure what causes this issue, nor do I know if it is super important. But it's there.