Bizarre caching of "dead" DNS entries; where is this cache stored?

Our investigation with OP finally narrowed to the particular file that is responsible for keeping mysterious dead entries.

The file that keep results of previous scans located in the file advanced_ip_scanner_MAC.bin that in turn stored in %AppData%\local\temp\Advanced Port Scanner 2. (It keeps text as Unicode characters, so not trivial to find if one searching for ASCII strings.)

Advanced IP scanner uses this file to show disconnected devices that were found with previous scans as dead entries. It could be useful if one need to wake up over WOL some devices, but it could be annoying if you travel between job sites and see such dead entries from others network, so if you don't need to keep results of previous scans, simply delete or rename this advanced_ip_scanner_MAC.bin file