capture_pane hangs with -S-

[deliciouslytyped]

I'm trying to expand the capture_pane functionality.
I'm using libtmux master on python3.7 with tmux 3.0 -
The following snippet hangs on some panes, while the same command works when run from bash;

>>> import libtmux; s = libtmux.Server(); s.children[0].children[0].children[1].cmd("capture-pane", '-p', '-J', '-e', '-E-', '-S-').stdout 
<omitted command noise>
DEBUG: ['/etc/profiles/per-user/myuser/bin/tmux', 'capture-pane', '-t', '%4', '-p', '-J', '-e', '-E-', '-S-']
<no output till ^C>

(I added some simple print debugging.)

list-panes gives the following:

$ tmux list-panes -t 0:
0: [135x17] [history 1952/2000, 939964 bytes] %0
1: [135x17] [history 928/2000, 431099 bytes] %4
2: [135x21] [history 1869/2000, 1194568 bytes] %5
3: [135x21] [history 0/2000, 0 bytes] %7
4: [271x13] [history 1816/2000, 269415 bytes] %1 (active)
5: [271x12] [history 6/2000, 1537 bytes] %132

The hang only seems to happen if I pass the -S- argument.

[deliciouslytyped]

Sorry this issues was supposed to go in libtmux.