@tatsumoto >Using proprietary software to manually enable the .ass file instead of using free software to embed the subs into the mkv with this script I found;
#!/bin/bash
# Copyright © 2024 DiffieHellman
# This program is free software: you can redistribute it and/or modify
# it under the terms of the GNU Affero General Public License as published by
# the Free Software Foundation, either version 3 of the License, or
# (at your option) any later version.
#
# This program is distributed in the hope that it will be useful,
# but WITHOUT ANY WARRANTY; without even the implied warranty of
# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
# GNU Affero General Public License for more details.
#
# You should have received a copy of the GNU Affero General Public License
# along with this program. If not, see <https://www.gnu.org/licenses/>.

# Return invalid usage if there aren't at 2 args past argv[0]
[ "$#" -eq 2 ] || exit 2

video=$1
subs=$2
# Map all streams from video file in argument and map all subs from sub file
ffmpeg -i "${video}" -i "${subs}" -map 0 -map 1:s -c copy "${video%.*}.mkv"