The issue seems to be due to a buggy Bios that was shipped with these laptops. I've also have an Acer Spin 1 (sp111-31) and have always had issues getting Ubuntu installed or doing grub updates for new kernels.
The issue along with workarounds are described here: https://gist.github.com/volcanoauthors/c7c9480dc78f1ba55d589d2125db4832
I tried updating my Spin 1 SP111-31's bios from the version that was shipped with the laptop (v1.02) with the latest from acer.com (v1.11). I used the feature where they ask for your laptop's serial number and got directed to https://www.acer.com/za-en/support/product-support/SP111-31/NX.GL5EA.004/downloads. You probably want to do the same (insert your serial number) to avoid getting a potentially incompatible BIOS.
The bad news is that the solutions for flashing BIOS without Windows all looked complicated and scary, so I gave in, and did a sacrificial Windows 10 install just to flash the bios. That went off without a hitch, simply unzipped the downloaded file and ran the executable. I did use the WoeUSB tool from the webupd8 ppa to create bootable windows install media in linux using the ISO I downloaded from Microsoft. Remember, you can install Win10 without creating a MS account if your system is not connected to the internet while installing.
The good news is that the new BIOS seems to fix the problem -- I can now install Ubuntu using the default installer with no further manual hacks, and grub updates normally when I install new kernels.
NB!!
Obviously installing windows will or may trash your system, so make backups and be sure that you know how to restore them. Or physically put a different drive into the laptop for windows, which is what I did.