An image rotated by 90° will not be found by the similarity search. This search was build for the purpose to find similar images. Not looking for images rotated by 90° or -90% or flipped or mirrored.
It is also one of the least used features in IMatch. It is available since IMatch 3, and it does well for what it has been designed. It has a rather low priority for me. Since I have only one spare time developer resource, I concentrate on things actually used and helpful for the IMatch user base.
If you are in need of a better visual similarity search and you have exhausted all the options available for this search feature, I recommend licensing a dedicated software for this purpose.
When I recall correctly, Google offers something in this areas, as well as Microsoft, Amazon and other cloud providers.
I neither have the technical expertise nor the thousands (!) of developer resources these IT companies have.
This is not something I can "improve easily". This is an area of ongoing high-end research carried out by universities, start-ups and the big IT companies because they can monetize the results. Maybe you can find something in the open source. Else you will have to find a suitable subscription and service for your needs.
From your posts, I understand that you have a very peculiar and probably unique workflow. Maybe IMatch cannot do everything you want, or not always in the way you want it. That's normal. You may need to combine multiple applications and services to implement every phase of your workflow. From Windows Explorer to specialized AI image searches.