SUMMARY
When using the Application Dashboard, pressing "Enter" on Plasma 5 would've opened the first application displayed. In Plasma 6, it takes a newline, so long as some text has already been entered. Pressing "Enter" with no search term will do nothing. Sometimes, search is functional across multiple lines, but it's inconsistent, and I don't think there's any benefit to searching for applications with a newline. If this is intended behaviour, an option to disable this would be appreciated!

There are some other strange things you can do, such as clicking around on the "Type to search..." text and pressing "Enter" to space it out. For example, clicking to put the cursor between "Type" and "to" and pressing Enter will allow you to take a newline. The same is also true for the "Searching for 'blah'" text. You can interfere with it in unexpected ways. I don't think this part, at least, is intended behaviour :-)

I'm not sure if the previous Plasma 5 behaviour is quite as applicable here, since searching now always groups applications by categories. But in that case, pressing "Enter" probably shouldn't do anything.

An option to return to the previous "unorganized" Dashboard search would be nice, but I can file a separate issue for that. This would make the Application Dashboard more consistent with the Application Launcher, which during a search, does not show categories and selects the top result by default. I could use the Application Launcher, but I vastly prefer the Application Dashboard :-)


STEPS TO REPRODUCE
1. Open Application Dashboard and enter some text
2. Press "Enter"
3. A newline will be taken, and you can take as many as you like, and it will eventually draw over the Applications list.
4. Click around on either the "Type to search..." text or the "Searching for" text
5. A cursor will be placed where you click, which is likely unexpected in itself
6. Press "Enter"
7. You can interfere with the text in funny, yet impractical ways :-)


OBSERVED RESULT
You can take a newline when searching using the Application Dashboard, and likewise take newlines when clicking to move the cursor around on the search text.

Previously, pressing "Enter" would open the first search result, i.e. searching for "Fire" and pressing Enter would open Firefox on my system.


EXPECTED RESULT
Pressing Enter will either do nothing as a result of the changes to the Application Dashboard, or open the first search result. The latter may not be intuitive behaviour anymore, as Application Dashboard now shows applications grouped by categories even during a search, instead of showing an "unorganized" search.


SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 6.7.8 Linux Zen
KDE Plasma Version: 6.0.1
KDE Frameworks Version:  6.0.0
Qt Version: 6.6.2 

ADDITIONAL INFORMATION
I filed this under "I don't know" because I couldn't find the correct place to report bugs with the Application Launchers, sorry...