A discussion of single engine IFR should include total engine failures for any cause, which isn't even tracked.
The relevant metric is called IFSD (engine in-flight shutdown) which is tracked and reported by law. E.g., for ETOPS operations the FAA requires that the 12-month rolling IFSD rate for the aircraft type/fleet worldwide must be less than limits set in 14 CFR 21.4 (ETOPS Reporting Requirements).
An engine event is considered an IFSD if it ceases to operate or is shutdown in flight for any unplanned reason (internal failure, icing, bird ingestion, etc.).
The mean time between IFSDs for PT6 is 125,000 hours. This is calculated for the population. E.g., if you have a fleet with 100 PT6s operating, then expect an IFSD over the entire population average 1,250 hr.
The 346,000 hours is actually not the MTBF of PT6s, but the mean time between engine failure
accidents for PT6 equipped Cessna 208 Caravans.
This does not mean a particular Caravan will make it 346,000 hrs before an engine accident. In fact, only a minority (1/e = 1/2.71) = 37% of Caravans can be expected to reach this amount of hours without an engine accident.