'set debug lin-lwp 1' seems broken with GDB/MI

On x86_64 GNU/Linux target, 'set debug lin-lwp 1' works as expected in CLI mode...

$ ~/.local/gdb-8.1.50/bin/gdb -q
(gdb) set debug lin-lwp 1
(gdb) attach 30460
Attaching to process 30460
sigchld
LNA: waitpid 30460, saving status Stopped (signal) (stopped)
PTRACE_ATTACH LWP 30461, 0, 0 (OK)
[New LWP 30461]
PTRACE_ATTACH LWP 30462, 0, 0 (OK)
sigchld
sigchld
[New LWP 30462]
LNSL: running -> suspending LWP 30462
LNSL: running -> suspending LWP 30461
LNSL: already stopped/no stop_requested yet LWP 30460
linux_nat_wait: [process -1], [TARGET_WNOHANG]
RSRL: NOT resuming LWP LWP 30462, not stopped
RSRL: NOT resuming LWP LWP 30461, not stopped
RSRL: NOT resuming LWP LWP 30460, has pending status
LLW: enter
LLW: Using pending wait status Stopped (signal) (stopped) for LWP 30460.
LLW: exit
LNSL: running -> suspending LWP 30461
linux-nat: already stopping LWP 30461 at GDB's request
LNSL: running -> suspending LWP 30462
linux-nat: already stopping LWP 30462 at GDB's request
linux_nat_wait: [process -1], []
RSRL: NOT resuming LWP LWP 30462, not stopped
RSRL: NOT resuming LWP LWP 30461, not stopped
RSRL: NOT resuming LWP LWP 30460, not resumed
LLW: enter
LNW: waitpid(-1, ...) returned 30462, ERRNO-OK
LLW: waitpid 30462 received Stopped (signal) (stopped)
LLW: resume_stop SIGSTOP caught for LWP 30462.
LNW: waitpid(-1, ...) returned 30461, ERRNO-OK
LLW: waitpid 30461 received Stopped (signal) (stopped)
LLW: resume_stop SIGSTOP caught for LWP 30461.
LNW: waitpid(-1, ...) returned 0, ERRNO-OK
RSRL: NOT resuming LWP LWP 30462, has pending status
RSRL: NOT resuming LWP LWP 30461, has pending status
RSRL: NOT resuming LWP LWP 30460, not resumed
SEL: Found 2 events, selecting #0
LLW: exit
sigchld
sigchld
sigchld
RSRL: NOT resuming LWP LWP 30462, not resumed
RSRL: NOT resuming LWP LWP 30461, has pending status
RSRL: NOT resuming LWP LWP 30460, not resumed
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib64/libthread_db.so.1".
linux_nat_wait: [process -1], []
RSRL: NOT resuming LWP Thread 0x7fba253fa700 (LWP 30462), not resumed
RSRL: NOT resuming LWP Thread 0x7fba25bfb700 (LWP 30461), has pending status
RSRL: NOT resuming LWP Thread 0x7fba26cb1740 (LWP 30460), not resumed
LLW: enter
LLW: Using pending wait status Stopped (signal) (stopped) for Thread 0x7fba25bfb700 (LWP 30461).
LLW: exit
0x00007fba25fba8ad in pthread_join () from /lib64/libpthread.so.0

...but not with MI:

$ ~/.local/gdb-8.1.50/bin/gdb -q -i=mi2
=thread-group-added,id="i1"
=cmd-param-changed,param="auto-load safe-path",value="/"
(gdb)
-gdb-set debug lin-lwp 1
^done
(gdb)
-target-attach 30460
=thread-group-started,id="i1",pid="30460"
~"../../gdb/ui-file.h:59: internal-error: virtual void ui_file::write_async_safe(const char*, long int): write_async_safe\nA problem internal to GDB has been detected,\nfurther debugging may prove 
unreliable.\nQuit this debugging session? (y or n) "

Observed on 8.1 release as well.

Dmitry