"While we cannot speak to the leading methodology of the survey, I can confirm that we have seen an increase in new member acquisition since we announced the updates to our Starbucks loyalty program last week," a Starbucks spokeswoman told CNBC.

Prior to the dip, Starbucks' scores had been trending near their highest levels in the recent past.

The company's "buzz" score — a measure of positive or negative sentiment around a brand — dropped from 60 to 29 in just eight days, according to YouGov BrandIndex. (Scores range from -100 to 100 and are gathered from people who have made Starbucks purchases in the last 30 days.)

Public sentiment on Starbucks plunged after the company revamped its loyalty program, suggesting the coffee chain may have an uphill battle with its most frequent customers.

"Buzz is a very objective measure of what have you heard and do you think of that as being positive or negative," Ted Marzilli, CEO of BrandIndex, told CNBC. "Consideration is is that going to potentially impede your willingness to go back and repurchase from that brand. We are seeing a more modest trend there, but a negative trend nonetheless."



On Feb. 24, some 80 percent of Starbucks customers said that they would consider making their next purchase at the coffee shop. That figure has fallen to 71 percent since the company tightened its rewards program, giving points to coffee drinkers for every dollar spent instead of redeemable points for every purchase.

"What we may be seeing is just a little bit of a disconnect between what Starbucks thinks the new program is about and what customers, in general, are perceiving of the new program," Marzilli said, explaining that many airline companies have updated rewards programs to benefit those that spend more money instead of those that spend more frequently.

Those that spend frequently, but pay less per purchase, are likely to balk at the new rewards program, while customers who tend to spend more per purchase will appreciate the switch.

"I'll tell you that this is a little bit concerning for Starbucks," Marzilli added, "but maybe they are hitting the right subset of customers and improving their perception of the brand."