Why my mask can't select the subject

Please post your System Information as Lightroom Classic (LrC) reports it. In LrC click on Help, then System Info, then Copy. Paste that information into a reply. Please present all information from first line down to and including Plug-in Info. Info after Plug-in info can be cut out as that is just so much dead space to us non-Techs and it takes up vast amounts of scroll space making the reply less readable and less likely that others will bother with your post.

Mostly interested in this part:

Graphics Processor Info: 
DirectX: NVIDIA GeForce RTX 3070 (31.0.15.4601)
Init State: GPU for Export supported by default
User Preference: Auto

My LR 13.0.2 does a much better job of identifying Subject using your second screenshot. Though Subject seems to be the least accurate of the AI masks, my experiment, combined with your screenshot showing the rectangular mask, strongly suggest your graphics driver may be out-of-date.

Most problems with LR's AI commands are caused by older, buggy graphics drivers. Try updating your graphics driver by going directly to the manufacturer's web site:
https://helpx.adobe.com/lightroom-classic/kb/troubleshoot-gpu.html#solution-4

If that doesn't help, please copy/paste here the entire contents of the LR menu command Help > System Info -- that will let us see exactly which versions of hardware and software LR thinks you're running.