I found a bit of time to do a test (basically because I needed access to certain notes) and ran the same search on Evernote on my iPhone as well as on my computer running macOS, and I got different result. I found 6 notes with a specific search query on my phone, and 0 notes with exactly the same search query on the computer.
Both devices were connected to the Internet. While I understand that only the server is guaranteed to contain my complete collection of notes, there seems to be an error in the macOS version, in either the caching of local notes or search index information on macOS, or in the way the server search functionality is used.
However, I tried by installing a very old version of Evernote (version 7) which still runs fine for a colleague of mine, and this also doesn't work for me. I tried to delete some cached files in my user's library folder on my mac, as I suspected there may be an issue with those, but this didn't help.
I will try to continue using it on my iPad and iPhone, hoping to avoid the issue on macOS for now, even though I ran out of the amount of switches I can make to different devices since I'm using the free version of Evernote. I'd be happy to becoming a paying customer if it offered any specific value, but starting a paid subscription as a workaround for a bug in the free version doesn't really feel right...