consul upgrade election error

[YQyangqian]

when i follow the recommended consul-upgrade steps, i have the following doubt:

At the beginning, i have 3 consul server(A/B/C) with one in bootstrap mode(A).

when i run 'consul leave' command in the server with bootstrap-mode(A), the remaining 2 servers will elect new leader(B). no error here.
when i restart A with bootstrap mode, there comes the error.
should i remove the bootstrap option where i restart?