Why On Load and Document complete differ?

Hi all!
I have faced the issue that ‘On Load’ differs from ‘Document complete’ significantly. I have a third party script loaded asynchronously which makes several calls but it only affected FPL, and “Document Complete” was equal to “On Load”. Currently these metrics are different ~3s (DC) vs ~1s (OL). On this forum there is a small thread regarding it https://www.webpagetest.org/forums/showthread.php?tid=14383&highlight=Document+Complete+on+load but I do not really get the phrase “They should be the same but if there are several navigations it may trip something up.”

The problem emerged for me on Monday 9/9/2019 and basically Document Complete starts to wait until all the requests of the 3rd party are executed and all DOM manipulations are done in results.

Is it an expected behavior? Were there any changes applied For WPT tool for how Document Complete is measured?

Thank you in advance!