`bind_rows<fable>()` fails when index column position differs

[mitchelloharawild]

library(fable)
#> Loading required package: fabletools
library(dplyr)
#> 
#> Attaching package: 'dplyr'
#> The following objects are masked from 'package:stats':
#> 
#>     filter, lag
#> The following objects are masked from 'package:base':
#> 
#>     intersect, setdiff, setequal, union
fc <- as_tsibble(mdeaths) %>% 
  model(SNAIVE(value)) %>% 
  forecast()
fc %>% 
  mutate(.model = "a") %>% 
  relocate(.mean) %>% 
  bind_rows(fc)
#> Error: Unsupported index type: character

^{Created on 2020-08-05 by the reprex package (v0.3.0)}

[mitchelloharawild]

The invalid tsibble structure is produced in this operation:

#vctrs::vec_cast_df_fallback_normalise()
  x[seq_along(to)] <- cast
  names(x) <- names(cast)

https://github.com/r-lib/vctrs/blob/bc1abf88f755828cd0c266cad562a1443b312d65/R/type-data-frame.R#L341-L345