Unable to delete some specific photos on my iPhone that were sync'd from my MacBook

You can remove them by turning off syncing of those photos. If they were in an album with other synced photos that you do not want to remove, you will need to remove the entire album, rearrange on your computer and then sync the rearranged album.

See the following from Keep your photos safely stored and up to date on all of your devices - Apple Support

If you don't want the synced photos on your iPhone, iPad, or iPod touch anymore, turn off syncing in iTunes and they'll be removed from your device. You can turn off syncing for some of your photo albums or all of them at once.

Stop syncing some albums

1. Open iTunes on your computer and connect your iPhone, iPad, or iPod touch.
2. Click on the device icon in iTunes.
3. Click Photos.
4. Choose "Selected albums" and deselect the albums or collections that you want to stop syncing.
5. Click Apply.

Stop syncing all photos

1. Open iTunes on your computer and connect your iPhone, iPad, or iPod touch.
2. Click on the device icon in iTunes.
3. Click Photos.
4. Deselect "Sync Photos" then click "Remove photos."
5. Click Apply.

The only other way that I know of is the last item in my previous reply (the reference to Léonie) -- that should get rid of the synced photos but not other photos, but it does involve turning on iCloud Photos.

According to the following post by Léonie, you may need to sync one more time. She also has another trick involving iCloud Photos. See: Previously synced photos not deleting - Apple Community