What happened

Shares of Duolingo (DUOL 0.62%) gained 1.3% in September, according to data from S&P Global Market Intelligence. The price increase isn't impressive in itself, but the language-learning expert achieved this modestly positive return in a month when the S&P 500 index fell 9.3%. In that context, Duolingo's stability stood out like a bucket of water in the desert. 

So what

The company didn't have a ton of news to share last month. CEO Luis von Ahn popped up in newspapers and social media feeds a couple of times, discussing his Guatemalan background and Duolingo's stance on potential abortion bans in its home state of Pennsylvania, but these events didn't move the stock.

Instead, shares soared on Sept. 9 and Sept. 28, two of the general market's strongest days in this period. The S&P 500 rose 1.9% and 1.5% on those two days, respectively, while Duolingo notched gains of 7.3% and 4.5%.

As a formerly high-flying growth stock, Duolingo was always hypersensitive to the market mood, rising and falling quicker than most of its peers. Hence, it was primed to bounce back faster than the general market last month. Heading into September, Duolingo shares had fallen 27.4% over the previous 52 weeks, far deeper than the S&P 500's 12.6% dip in the same period.

Now what

Never mind the plunging stock price; Duolingo's business never cooled down, which is why it makes sense to see some color in the stock's proverbial cheeks again.

In early August's second-quarter report, daily active users increased by 44%, and the number of paid subscriptions rose 71%, compared to the year-ago period. As a result, revenue soared 50% and free cash flows surged from $2.1 million to $9.9 million. In short, the company is pulling in scores of users and paying subscribers, which also adds up to tremendous gains on the top and bottom lines.

Of particular interest, Duolingo's management never mentioned the word "inflation" in the second-quarter report and earnings call. The issue that is weighing on this company's stock simply isn't a problem worth mentioning. If anything, Duolingo's shares should be rising in 2022.

But that's not how market makers approach the stock these days, so we had to settle for a month of robust stability instead. Shares aren't cheap by any stretch of the imagination, but there seems to be a mismatch between the company's tremendous business growth and the swooning stock chart. Therefore, Duolingo looks like a no-brainer buy right now.