Why don't you use the advanced search, then you'll find the coins you're searching for:
https://en.numista.com/catalogue/index.php?mode=avance&p=1&l=&r=&e=sri_lanka&d=&ca=3&no=&i=&v=&m=&a=1978&t=&dg=&w=&u=&f=&g=&c=&tb=y&tc=y&tn=y&tp=y&tt=y&te=y&cat=y
I don't understand either why you get that Nepalese coin. Of course you typed "Sri Lanka 1978" in the description (so not with the issuers, where "Sri Lanka" belongs), so the search is looking for the individual words "Sri" "Lanka" and "1978". That Nepalese coin gives a match for "Sri" (a symbol above the square between the crescent and the sun on the obverse) and for "1978", but there is nowhere "Lanka" in the description, so I don't know why the search find this coin.
But it's logical you don't find any coins from Sri Lanka in your search, because the country name is nowhere in the description on the coin page (except in the "Features" box, but I guess this box is excluded from the search").
