@Remus2I gave examples of when a hyphenated (compound) noun turns into a word without the hyphen, but I didn't address all of your question โ that is, "round trip" was two words. Here's a Grammar Girl blog that discusses that:
https://www.quickanddirtytips.com/education/grammar/how-to-use-a-hyphen
The wind in Reno is so loud that I'm using my noise canceling headphones to do the audio editing on my podcast today.
@GrammarGirl do you mean "noise-canceling"? ;-)
A hyphen would be OK there, because "noise" and "canceling" are acting as a compound modifier, modifying "headphones."
Notice how I said it would be OK, and I didn't use any strong words like "must hyphenate" or "should hyphenate"? Although there are a few hard-and-fast rules for using hyphens, there are just too many exceptions to call everything relating to hyphens a rule.