“Major bugs” in Facebook’s political transparency tools are preventing researchers and journalists from holding advertisers to account, according to a report from Mozilla.

It comes as the Conservative party is spending thousands of pounds a day experimenting with new adverts on the social network, prominently emphasising the leadership of Boris Johnson.

Although the party’s spending habits are ostensibly disclosed by the social network, in practice it is hard to uncover specific details because of flaws identified by researchers at Mozilla, the non-profit foundation behind the Firefox browser.

Launched in the UK in 2018, the Facebook ad library is a public record of all the adverts related to social issues, elections or politics that have been posted on the site. In March, the social network launched an API – an application programming interface, or set of protocols – for the library, intended to let researchers perform complex analysis on the information contained within.

Instead, researchers found a series of bugs and design flaws that hampered any serious attempt to hold advertisers to account. “Once our technical experts began working with the API we quickly concluded it was unusable,” the foundation said.

“Through a month of rigorous testing we uncovered 12 major bugs, that plagued the tool and were reported to Facebook.

“Targeting information was not available, bulk data access was not offered, and the data wasn’t tagged properly. In fact, no search was alike. Repeating an identical search would produce wildly differing results. The state of the API made it nearly impossible to extract the data needed for researchers and journalists to ‘analyse ads’ as Facebook had claimed.”

Those flaws greatly limit the amount of information available about political advertisers on the social network. For instance, Facebook publishes a daily report about spending on the site, which reveals that the Conservative party spent £4,003 on 27 July, the most recent date for which data is available, and £13,487 during the previous week.

The spending is enough to make the party the second-largest political advertiser on Facebook, after the beer industry’s campaign to cut alcohol duty.

But because the Conservatives are testing a new message, and a new leader, they far outrank other advertisers in terms of the number of unique ads: in just one day, the party posted more than 600 unique variations of adverts, according to Facebook’s daily reporting. Each individual advert had less than £100 spent on it, and received fewer than 1,000 views, but in total, the adverts will have been shown to many thousands of recipients, providing valuable information for the party about which messages resonate well with various audiences.

“The ads are a bit of a mixed bag in terms of what they’re trying to get people to do,” said the online advertising campaign group Who Targets Me. “On one hand, they’re video ads designed to persuade and motivate. But at the same time you’re also asked to click through and fill out a survey.

“The Brexit party, still the overall highest political spender on Facebook over the last month, [appears to] have spent £0 [on Facebook] since Johnson became PM.”

Who Targets Me reported that the party’s spending had doubled in two days, from £2,000 on 25 July to £4,000 on 27 July. But to do so, the group had to keep its own offline records of advertising spending, since Facebook does not offer researchers historical data.

In response to queries about the bugs in its transparency tools, Facebook told the New York Times: “We were the first to introduce this level of ads transparency, and it remains a priority.” A spokesperson said the company “continually seeks feedback from researchers and journalists”.