This issue exists in both in 8.6.6 and revised_text.
The negative numbers should not be returned either.

package require Tk
pack [text .t -width 20 -height 10 -wrap char]
.t insert 1.0 "abcd\nefg hijkl mnop qrstuv wxyz"
.t tag configure y -wrap none
update
.t tag add y 1.end 2.2
.t bbox 2.20  ; # should return {}

% .t bbox 2.2 ; # g
18 18 7 14
% .t bbox 2.3 ; # { }
25 18 -7 14
% .t bbox 2.18 ; # t
130 19 -7 15
% .t bbox 2.19 ; # u
137 19 -7 15
% .t bbox 2.20 ; # v  off-screen
144 19 -7 15
% .t bbox 2.21 ; # { } off-screen
151 19 -7 15
% .t bbox 2.22 ; # w
%