The sustainability of Open Source Software (OSS) projects often depends on a small group of core contributors. When these contributors disengage, projects may face disruption; however, the consequences of such disengagement on OSS contribution workflows remain underexplored. This paper presents a large-scale quasi-experimental study quantifying the impact of core contributor disengagement on pull request (PR) throughput, acceptance rate, and time to merge. We analyzed over 35 million PRs across 50,804 GitHub repositories, identifying 95,958 disengagement bursts (one or more core contributors becoming inactive for >365 days within the same week) involving 174,887 contributors. Using Difference-in-Differences with matched control groups, we found that the impact varies substantially with contributor and project characteristics. Disengagements of contributors with a high share of commits lead to pronounced declines in throughput and acceptance, while long-tenured contributors’ disengagements have milder effects on those metrics but increase merge time, suggesting loss of tacit project knowledge. These findings provide empirical evidence of how core contributors’ disengagement influences OSS workflows and highlight structural factors associated with project resilience.