You create one page in a first language and you publish it. Google crawls the page displayed without hreflang attributes because the page is not translated yet.
Later, you translate the page in a second language and publish the translation. Google crawls the translation and finds hreflang attributes since the page is translated. Google has crawled the first page and its translation and reports an error about hreflang attributes not matching.
Even later, Google crawls the first page again and finds the hreflang attributes. They are matching the hreflang attributes of the translated page. Google doesn’t report the error anymore. Please be patient. It can take time for Google to crawl your original page after you translated it.
You would have the same error if you originally have a page in several languages and add later an additional translation. In this case, the error may be reported during even more time as all translations must be crawled by Google before it removes the error.