The only words in Russian ending by олнц are derivatives from "солнце". In all the rest of the words with олн + vowel л is alway pronounced. I'm aware only of two words with олн + consonant, that are:
- чолнт - л is pronounced in this word.
- Солнзас (or Солнзап) - it's a river and as far as I know
лis pronounced there.
So yes, that's an exception. But, strictly speaking, statement "in all Russian words with 'олнц' л is always omitted" will be valid.
As a side-note - there are quite a lot of words where one should memorize the Russian pronunciation - in сердце д is mute, in грустно т is mute, in здравствуй first в is mute. I can go on for a while - it's not as crazy as in English - but still. Here's a link that might be useful for you.