Using /spark sampler = Profiler now active! (async)
Using /spark sampler --timeout 300 = Profiler now active! (built in java)

Not sure if this is intended or not but it seems that when you specificy a timeout value that the async engine is not used. Is there any way to force it or is this a bug/intended?

Running Sponge API 7

i have that issue too. im using forge tho.
also the async results are empty.

The async profiler engine doesn't support timeouts - so, yep, it's intended :)