Wrapper uses wrong tracking data in email when sending fast over multiple connections

[SendFlo]

When delivering many emails (>10/second) over multiple connections (8+), identical tracking data from one email ends up in other emails.
This might happen because all "Session"s share the same "Backend" and "Wrapper" struct.
I assume that the WrapperData gets overwritten by a parallel process before the data can be copied upstream.
I tried making a copy of the wrapper for each new Session, but it sadly did not solve the issue.