I can sadly attest to this behavior as recently as a couple years ago :(

I searched for a domain that I couldn't immediately grab (one of more expensive kind) using a random free whois site... and when I revisited the domain several weeks later it was gone :'(

Emailed the site's new owner D: but fairly predictably got no reply.

Lesson learned, and thankfully on a domain that wasn't the absolute end of the world.

I now exclusively do all my queries via the WHOIS protocol directly. Welp.