E-commerce giants like Alibaba Group and JD.com have historically used the weekslong 618 shopping festival to get customers to open their wallets, offering promotions on everything from Coach handbags to iPads. This time around, JD said it posted record gross merchandise value, the total value of goods sold on its platform. A person familiar with Alibaba’s sales said the company’s GMV beat last year’s tally.

But given the unusually steep discounts this year—the product of cautious Chinese consumers and rising competition from fast-growing low-cost leader PDD Holdings—analysts are skeptical that companies significantly boosted their bottom lines.

“We believe GMV may have slowed down” during the second half of the 618 event, HSBC analysts wrote in a research note after sales wrapped up last week. They cited official data showing that on-year parcel volume growth slowed from a percentage in the high 20s at the start of the sales event to the high teens in the final week.

Margins at Alibaba’s major e-commerce platforms Taobao and Tmall, they added, “could see a drag.”

Citi analysts led by Alicia Yap said one of the key takeaways of the event this year was that merchants appeared to be facing more intense competition and pressure on margins.

“A very obvious trend this year is low-price competition,” as companies like Alibaba and JD learn from PDD’s method of capturing market share via low prices, added Shawn Yang, senior research analyst at Arete Research. “Profitability might be affected, but it’s hard to quantify it at this point.”

Analysts stopped short of calling the sales a bust. Complicating analysis is the fact that major companies didn’t provide tallies for total sales, a departure from several years ago when they regularly announced record turnovers. Third-party market data providers were divided on the results, with Syntun saying the combined GMV across major traditional online e-commerce platforms fell 7% on the year to the equivalent of about $78.69 billion, while Analysys estimated that GMV rose 11%.

HSBC said divergence among leading data collectors was likely due to changes in promotional approaches and app functions by sellers that may have undermined the usual ways of deriving trends based on data scraping.

Analysts have been looking for signs of a revival of consumption in China, the world’s second-biggest economy, where shoppers have cut back on spending amid a lackluster recovery from the Covid-19 pandemic and pressures from the property sector’s collapse. China’s economy expanded 5.2% in 2023, one of the lowest levels in decades.

More recent data showed that retail sales, a key gauge of consumer spending, rose 3.7% in May from a year ago, up from a 2.3% gain in April, but economists have cautioned that further improvement is necessary for meaningful economic recovery.

Shares of Alibaba and JD, two of China’s biggest tech companies, have fallen 13% and 23%, respectively, over the past 12 months amid slowing sales and other pressures.

PDD stock, on the other hand, has doubled as price-conscious consumers flocked to the company’s Pinduoduo platform for deals. PDD’s revenue surged 131% from a year earlier in the first quarter, compared with single-digit growth at Alibaba. Analysys estimated that PDD’s GMV rose 18% on the year during the 618 event, outperforming estimated growth of 12% at Alibaba’s major platforms and 5.7% at JD.

“The whole China e-commerce industry is now embracing the tactics created by the industry’s poster child, Pinduoduo, which has successfully branded itself as a platform that offers low prices every day,” Nomura analysts Jialong Shi and Rachel Guo said in a research note. “This has taken away the appeal of shopping festivals like June 18, in our view,” they wrote.

Write to Tracy Qu at tracy.qu@wsj.com