I'm reopening this as I found another occurrence. Here's the backtrace:
Thread 1 "emacs" hit Breakpoint 1, signal_or_quit (error_symbol=..., data="" keyboard_quit=false) at eval.c:1807
1807 {
#0 signal_or_quit (error_symbol=..., data="" keyboard_quit=false) at eval.c:1807
#1 0x00005600b7049b6b in quit () at eval.c:1797
#2 0x00005600b7049a49 in process_quit_flag () at eval.c:1744
#3 0x00005600b7049a95 in maybe_quit () at eval.c:1764
#4 0x00005600b705c223 in Fdelq (elt=..., list=...) at fns.c:1845
#5 0x00005600b706cf49 in font_put_extra (font=..., prop=..., val=...) at font.c:749
#6 0x00005600b70757ca in font_clear_prop (attrs=0x7ffd8a5f1620, prop=FONT_WEIGHT_INDEX) at font.c:3148
#7 0x00005600b6ef3b0d in merge_face_vectors (w=0x5600b8e523a0, f=0x5600b8e52160, from=0x7ffd8a5f1370, to=0x7ffd8a5f1620, named_merge_points=0x7ffd8a5f1350) at xfaces.c:2193
#8 0x00005600b6ef4197 in merge_named_face (w=0x5600b8e523a0, f=0x5600b8e52160, face_name=..., to=0x7ffd8a5f1620, named_merge_points=0x7ffd8a5f1350, attr_filter=0) at xfaces.c:2309
#9 0x00005600b6ef5afe in merge_face_ref (w=0x5600b8e523a0, f=0x5600b8e52160, face_ref=..., to=0x7ffd8a5f1620, err_msgs=false, named_merge_points=0x0, attr_filter=0) at xfaces.c:2793
#10 0x00005600b6ef38e4 in merge_face_vectors (w=0x5600b8e523a0, f=0x5600b8e52160, from=0x7ffd8a5f16c0, to=0x7ffd8a5f1620, named_merge_points=0x0) at xfaces.c:2168
#11 0x00005600b6efc11b in lookup_derived_face (w=0x5600b8e523a0, f=0x5600b8e52160, symbol=..., face_id=4, signal_p=false) at xfaces.c:5006
#12 0x00005600b6df679f in handle_single_display_spec (it=0x7ffd8a5f3060, spec=..., object=..., overlay=..., position=0x7ffd8a5f31b0, bufpos=7819, display_replaced=0, frame_window_p=true, enable_eval_p=true) at xdisp.c:5825
#13 0x00005600b6df4858 in handle_display_spec (it=0x7ffd8a5f3060, spec=..., object=..., overlay=..., position=0x7ffd8a5f31b0, bufpos=7819, frame_window_p=true) at xdisp.c:5472
#14 0x00005600b6df3ff8 in handle_display_prop (it=0x7ffd8a5f3060) at xdisp.c:5380
#15 0x00005600b6def7ab in handle_stop (it=0x7ffd8a5f3060) at xdisp.c:3879
#16 0x00005600b6e00d65 in next_element_from_buffer (it=0x7ffd8a5f3060) at xdisp.c:9126
#17 0x00005600b6dfce4c in get_next_display_element (it=0x7ffd8a5f3060) at xdisp.c:7713
#18 0x00005600b6e3405f in display_line (it=0x7ffd8a5f3060, cursor_vpos=21) at xdisp.c:23907
#19 0x00005600b6e2437d in try_window (window=..., pos=..., flags=1) at xdisp.c:19811
#20 0x00005600b6e20f58 in redisplay_window (window=..., just_this_one_p=false) at xdisp.c:19218
#21 0x00005600b6e181e9 in redisplay_window_0 (window=...) at xdisp.c:16929
#22 0x00005600b7049210 in internal_condition_case_1 (bfun=0x5600b6e181a3 <redisplay_window_0>, arg=..., handlers=..., hfun=0x5600b6e18167 <redisplay_window_error>) at eval.c:1516
#23 0x00005600b6e18138 in redisplay_windows (window=...) at xdisp.c:16909
#24 0x00005600b6e180ea in redisplay_windows (window=...) at xdisp.c:16903
#25 0x00005600b6e16a6c in redisplay_internal () at xdisp.c:16377
#26 0x00005600b6e178bc in redisplay_preserve_echo_area (from_where=8) at xdisp.c:16730
#27 0x00005600b6f7d53b in detect_input_pending_run_timers (do_display=true) at keyboard.c:10457
#28 0x00005600b70c25ed in wait_reading_process_output (time_limit=30, nsecs=0, read_kbd=-1, do_display=true, wait_for_cell=..., wait_proc=0x0, just_wait_proc=0) at process.c:5697
#29 0x00005600b6dc379c in sit_for (timeout=..., reading=true, display_option=1) at dispnew.c:6154
#30 0x00005600b6f681d8 in read_char (commandflag=1, map=..., prev_event=..., used_mouse_menu=0x7ffd8a5f88ed, end_time=0x0) at keyboard.c:2801
#31 0x00005600b6f7b417 in read_key_sequence (keybuf=0x7ffd8a5f8ad0, prompt=..., dont_downcase_last=false, can_return_switch_frame=true, fix_current_buffer=true, prevent_redisplay=false) at keyboard.c:9631
#32 0x00005600b6f6384f in command_loop_1 () at keyboard.c:1393
#33 0x00005600b704912d in internal_condition_case (bfun=0x5600b6f633b1 <command_loop_1>, handlers=..., hfun=0x5600b6f627c2 <cmd_error>) at eval.c:1492
#34 0x00005600b6f62f72 in command_loop_2 (handlers=...) at keyboard.c:1134
#35 0x00005600b70482cc in internal_catch (tag=..., func=0x5600b6f62f47 <command_loop_2>, arg=...) at eval.c:1223
#36 0x00005600b6f62f12 in command_loop () at keyboard.c:1112
#37 0x00005600b6f6226d in recursive_edit_1 () at keyboard.c:721
#38 0x00005600b6f62489 in Frecursive_edit () at keyboard.c:804
#39 0x00005600b6f5e035 in main (argc=1, argv=0x7ffd8a5f8f48) at emacs.c:2409
It happens when the cursor in one window is on an overlay created by flycheck-mode, and I press C-g in another window.