No, LR has always operated this way. It stores all metadata in the .xmp sidecar, including develop settings. Consider this sequence of events:
1. Import into LR a raw with its Imatch .xmp containing keywords.
2. Make edits in LR.
3. Make changes to the keywords in IMatch and write them to the .xmp.
4. In LR, do Metadata > Read Metadata From File.
Step 4 will read in the new keywords from the .xmp in step 3, but since the develop settings from step 2 aren't in the .xmp, step 3 reads in empty settings, overwriting the changes you made in step 2.
If you inserted this step after making the edits in step 2:
2a. Do Metadata > Save Metadata To File
then the .xmp sidecar contains the develop settings before you edit the keywords using Imatch in step 3, and step 45 will reread the new keywords and the new develop settings.
Perhaps in LR 5 you had the set the option Catalog Settings > Automatically Write Changes Into XMP. This causes changes to metadata (including develop settings) to be written back to the .xmp sidecars very soon after the changes are made, obviating the need in most cases to manually save it with step 2a.
But if you choose to use that option and continue with your Imatch workflow, be aware that, while the changes are nearly almost always written back within a second or two, in rare cases it takes longer, especially if you've made changes to a large batch of photos. Before going back to make changes in Imatch, use a Library Filter bar preset or a smart collection to verify that all photos have Metadata Status = Up To Date.