You posted a feature request and while we’re waiting for a fix, you can try to correct your situation by doing the following:
Search for photos with keyword “Flowers”
Select all found and edit the keyword
Remove “Flowers” from the keyword list
Depending on how many photos you need to correct, it might be less effort to find the “Flower” photos, change these to “Flowers” and change “Flowers” to "Flower in the keyword list.
Note that PL on Mac can’t find more than 1000 photos. In this case, change the keyword of the photos and repeat until done.