<rdar://problem/14119118> When Smart Copy/Paste is off, double-clicking a word, whether in editable or non-editable text, also selects the following space. The space should not get selected along with the word regardless of whether Smart Copy/Paste is on or not.
If it helps with figuring this one out, it seems that toggling one of the flags in the Debug > Drawing/Compositing Flags menu (e.g. "Enable/Disable Subpixel-Antialiased Layer Text") will stop this behavior. Some of the flags seem to only work on a per-tab basis, others per-window.
I'd be very surprised if "Enable/Disable Subpixel-Antialiased Layer Text" affected this.
Created attachment 427303 [details] Video of toggling a flag to stop bug behavior Safari 14.1 (15611.1.21.161.5, 15611) on macOS 10.15.7 (19H1030)
Flipping *any* pref seems to do the trick.
This bug also affects Mac Mail app, which uses WebKit for editing.