Children spans are not visible in the timeline UI until parent span finishes

[zszepczynski]

Describe the bug
Elastic version 7.17.7
We are using open-telemetry traces if that matters.
We have service A calling service B. Service B starts a parent long running span that initiates whole transaction in that service. Under that span there are many short lived spans that are created and sent to elastic APM immediately when they finish. Unfortunately they are not visible in the Timeline (last span is from service A) until long running span finishes and gets sent to elastic APM. If the operation takes 30 minutes - all the spans from service B will be visible after 30 minutes, no earlier. I can't use it for live debugging.
If I look into "View Sample Document" I can see these spans being received earlier by elastic APM.

Expected behavior
I'd expect to see the traces without parent as they come in the Timeline. After parent span is finished, they would be properly grouped under that parent - similarly to what Jaeger does.