They will not notice it unless they try to make contact with you after it is done. But things can become awkward if you are both a part of the same group chat. You won’t see their messages but they will see yours. And as a rule, if they are blocked, they won't see any changes of yours.
Actually Android 6.0 had Full Disk Encryption as specified in the Android 6.0 Compatibility Definition Document - Section 9.9 Full Disk Encryption with exceptions if the hardware was insufficient to perform the operation.
So your case of a not specified lockscreen password is actually called out:
If the user has not specified a lockscreen passcode or ...
The following observation supports a conclusion that this no more than a quirk of the last system update:
According to this link to an Android package QMMI, it seems that this is created by HMD Global the company that produces Nokia.
In this page there is a section called 'Disabling verified boot' that mentions a commands to create an empty vbmeta.img that can be used to disable verified boot : https://wiki.postmarketos.org/wiki/Android_Verified_Boot_(AVB)
That command is,
$ avbtool make_vbmeta_image --flags 2 --padding_size 4096 --output vbmeta_disabled.img
To ensure this, I searched ...