First, run mprocs with just fed-shell. Then, generate a long string that spans multiple lines, select it, and then press "c" to copy it:

When I paste this string, I get the following text with linebreaks:

AAAAAAAAAAEAAAAAAAAAAQAAAAAAAAABCEugU0FOLagwr2NaNai2VNvDxe0vfNXss7bfggLQVK2wa1W5
PmtX7Dm72XyraGBSljmIadzJagAwofdsWXYDdPi0ErRGfwOs/Qp9D9RZYLGetUCm46xl3Z6dR0f55+RGSLQEJ9atIW9
JJmJOPYc/lg==

So I can't just paste the output back into the fedimint-cli reissue command without manually editing the linebreaks.

Asked this question upstream in mprocs repo, too.