There are two types of deflation. Natural deflation, which is the decrease in prices as we get better technology and better processes for producing goods and services, which is good. The other deflation is monetary contraction, which happens at the back end of central bank monetary expansion. This is bad.

The stagnation happens under the second type of deflation. Bitcoin causes the first type.

For more, I suggest you read Peter St. Onge's excellent summation of the topic: https://www.profstonge.com/p/the-deflation-is-bad-myth