It seems Verified Boot has been implemented by Fairphone
Can you please try to rebuild TWRP with these kernel defconfig flags disbaled
CONFIG_MODULE_SIG_KEY=“certs/signing_key.pem”
CONFIG_SYSTEM_TRUSTED_KEYRING=y
CONFIG_SYSTEM_TRUSTED_KEYS=""
You can follow this post here to build twrp yourself
For OS side you will have to unpack stock boot.img diff initscripts and fstab remove any verity checks here .
From your tree it seems to be okay https://github.com/mstaz/android_device_fairphone_fp3/blob/master/rootdir/etc/fstab.qcom#L36
I will my receive fairphone 3 in 1 week looking forward to join the development .