Windows File Explorer uses a proprietary metadata field for storing the Subject field shown in the Properties window. (Technically, it's the XPSubject field in the EXIF metadata section, where "XP" refers to Windows XP.)
LR generally adheres to the industry standard Guidelines For Handling Image Metadata published by the Metadata Working Group, which includes Adobe, Apple, Canon, Microsoft, Nokia, and Sony. This standard came out long after Windows XP, but I guess Microsoft hasn't thought it important to update Windows File Explorer to align with the standard.
If you want LR to be able to read all of the values you've entered in File Explorer's Subject field, you could use the free ExifTool utility on the command line:
$ exiftool '-MWG:Description<EXIF:XPSubject' *.jpg
This copies the XPSubject field to the industry-standard fields containing what LR calls "caption". If you've already imported these pics into LR, do these steps:
1. Select all the pics and do Metadata > Save Metadata To File
2. Run the ExifTool command above.
3. Select all the pics and do Metadata > Read Metadata From File
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And is there a way to fill in a box with "Subject" in Adobe Lightroom for multiple photos at once?
If by this you mean that you'd like multiple photos to all have the same contents displayed in the Caption field in LR's Metadata panel:
1. Select all the pics.
2. Go to the Metadata panel in the right column of Library.
3. Type what you'd like their new caption to be in the Caption field.