The WPFTS Pro version you are using is very outdated. Please consider upgrading to 2.46.x which is the latest. It has a completely new indexing and searching algorithm and it should solve your issue.
Default Search Logic (AND / OR) broken since Version 1.28.75
Under Settings / Filter
Default Search Logic
This option tells the search engine whether all query words should contain in the found post (AND) or any of these words (OR).
AND no longer works. It behaves the same as OR.
This worked in version 1.28.75 and has been broken since. I now have the latest version 1.34.94 which has this issue.
My Staging environment has 1.28.75 and my dev env. has 1.34.94 and my dev environment no longer searches correctly.
I have the latest version of wordpress 5.5 on both Staging and Dev, and when I initially upgraded from 1.28.75 to 1.30.85 that is when I noticed the issue. I also did a full reindex as well.
Switching between AND and OR had no effect
I have some a bunch of resources and in the sandbox when I search for "Frog" it finds 22 resources when I search for "Frog Organs" it finds 37, clearly finding more is OR logic. It should have narrowed down to 1 resource found.
Is this a known issue?
I've just tested 1.34.94 and really, I see the issue. Should be we lost something in the code. I will find the reason for the issue and fix it very soon.