Undefined Index error in scan logs

Hi
Ive found various threads about this in the last year, but they seem to end with people sending you logs but nobody ever saying what the problem was. I’ve just tried a ‘vanilla’ WordPress install, with a ‘vanilla’ WordFence plugin install, with only a handful of the most basic config changes, and no aditional plugins or content of any kind. I’m still getting these errors.
The purpose of the ‘vanilla’ install was to start trying to pin down a different issue entirely (see below) but I don’t have a lot of confidence in testing for that one if WordFence is throwing all these errors on the most basic install/config 🙁
Interestingly, looking up the line numbers shown in the errors, they relate to status issues – is this then related to the ‘stored data not found’ error (see below). That WordFence is trying to refer back to temporary files that, for some reason, are then not found, and throwing an error? Once again suspecting filesystem/server errors…?

Config is:
LiteSpeed server
PHP 7.0
Wordpress 4.9.6 + Twenty Seventeen (search engines discouraged), UK English
– subdirectory install but NOT reconfigured to hide that
– disabled comments & set home page to a new Page
WordFence 7.1.6
– Configured firewall & enabled auto-update
– Otherwise settings default

For reference, the error I was trying to troubleshoot was:
Scan can’t continue – stored data not found after a fork
https://wordpress.org/support/topic/scan-cant-continue-stored-data-not-found-after-a-fork-3/#post-10356865