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][1] - л is pronounced in this word.  
 - [Солнзас][2] (or Солнзап) - it's a river and as far as I know `л` is 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][3] that might be useful for you.   


  [1]: https://ru.wikipedia.org/wiki/%D0%A7%D0%BE%D0%BB%D0%BD%D1%82
  [2]: https://ru.wikipedia.org/wiki/%D0%A1%D0%BE%D0%BB%D0%BD%D0%B7%D0%B0%D0%BF
  [3]: https://orfogrammka.ru/%D1%82%D0%BE%D1%82%D0%B0%D0%BB%D1%8C%D0%BD%D1%8B%D0%B9_%D0%B4%D0%B8%D0%BA%D1%82%D0%B0%D0%BD%D1%82/%D0%BD%D0%B5%D0%BF%D1%80%D0%BE%D0%B8%D0%B7%D0%BD%D0%BE%D1%81%D0%B8%D0%BC%D1%8B%D0%B5-%D1%81%D0%BE%D0%B3%D0%BB%D0%B0%D1%81%D0%BD%D1%8B%D0%B5-%D0%B2-%D0%BA%D0%BE%D1%80%D0%BD%D0%B5/