That's odd. If unattended upgrades are running, the system will do upgrades regularly. That means it's unlikely to get a significant backlog of updates queued up. Upgrade cycles typically finish briefly as a result. All my systems, interactive or headless, are running an update and upgrade cycle every hour. I've yet to to run into a case when I couldn't install a package because apt was in use. It's not impossible, but I haven't. Or at least it's been so long ago that I've forgotten about it. I don't have to think about unpatched vulnerabilities. ☺️