BACKGROUND: Prosthetic valve dysfunction due to entrapment of the subvalvular apparatus is a rare condition, particularly when occurring as a delayed postoperative complication. Spontaneous papillary muscle rupture in the absence of an ischemic event is also uncommon. The simultaneous presence of both conditions renders this case of intermittent symptomatic mitral regurgitation exceptional. CASE PRESENTATION: We present the case of a 72-year-old woman with a history of rheumatic heart disease and a mechanical mitral valve prosthesis. Although the initial postoperative course was uneventful, she developed rapidly progressive heart failure symptoms two years after surgery, secondary to newly onset severe mitral regurgitation. Transesophageal echocardiography was crucial in elucidating the underlying mechanism, revealing a ventricular mass intermittently interfering with the proper closure of the mechanical prosthesis. The main differential diagnoses regarding the origin of the mass included: remnants of the subvalvular apparatus, vegetation, thrombotic material and pannus formation. Given the clinical deterioration, surgical intervention was considered necessary despite the lack of precise knowledge regarding the nature of mass. Surgical exploration confirmed the rupture of the subvalvular apparatus involving the anterolateral papillary muscle as the underlying cause of the prosthetic dysfunction. Excision of the ruptured chordae tendineae and residual papillary muscle was performed with a favorable outcome and no complications. CONCLUSIONS: This case illustrates a rare cause of late prosthetic mitral valve dysfunction: the entrapment of the subvalvular apparatus due to spontaneous rupture of the papillary muscle. Transesophageal echocardiography proved highly valuable in understanding the mechanism of dysfunction; however, surgical exploration ultimately established the definitive diagnosis and facilitated the correction of the issue. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13019-025-03521-8.