Temporal: Wrong offset when subtracting large duration from ZonedDateTime

[catamorphism]

 Temporal.ZonedDateTime.from("2001-09-08T18:46:40-07:00[America/Vancouver]").subtract(Temporal.Duration.from("P5432Y1837M")).toString()
-003584-08-08T18:59:08-08:00[America/Vancouver]
> Temporal.ZonedDateTime.from("2001-09-08T18:46:40-07:00[America/Vancouver]").subtract(Temporal.Duration.from("P5432Y1836M")).toString()
-003584-09-08T19:59:08-07:00[America/Vancouver]
> Temporal.ZonedDateTime.from("2001-09-08T18:46:40-07:00[America/Vancouver]").subtract(Temporal.Duration.from("P5432Y1835M")).toString()
-003584-10-08T18:46:40-08:12[America/Vancouver]

Notice that in the first result, the offset is -08:00; then -07:00; then -08:12.

The third result is consistent with the reference implementation; the first and second aren't. Given how early this date is, the offsets shouldn't be changing! (The reference implementation interprets this time zone as -08:12 prior to the introduction of standard time zones in 1883.)