It seems that while modeling in the Editor, when using Pools, message flows (dotted limes) connecting elements change into sequence flows (solid lines). This can be observed when, after connecting a task to another task with a message flow (dotted line), adding another task to the pool causes the switch to a sequence flow (solid line).
This is behavior is an auto-correction and is intended behavior. To give some more background from BPMN standards: Message flows cannot be used within the same pool. They are exclusively for EXTERNAL communication (pool-to-pool). Hence it always MUST be a sequence flow between tasks. That's also the reason, why we don't allow users to manually transform a message flow into a sequence one between tasks - it's simply not allowed according to BPMN. Please note, the auto-correction works also in both ways. Whenever task gets connected via sequence flow with one, in another pool, it automatically converts the sequence flow into a message flow.
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