Unexpected line number at BASH_LINENO[i]

[Michal Berger]

BASH_VERSION: 5.1.4(1)-release (shipped with Debian 11)

Considering script like this:

1       #!/usr/bin/env bash
2
3       set -o errexit
4       set -o errtrace
5
6       bt() {
7               local line file file_v
8
9               for ((idx = 1; idx < ${#FUNCNAME[@]}; idx++)); do
10                      read -r line _ file < <(caller $((idx - 1)))
11                      file_v=${file//[\/\.]/_}
12
13                      [[ -f $file ]] || continue
14
15                      if [[ ! -v $file_v ]]; then
16                              mapfile -t "${file_v}_ref" <"$file"
17                      fi
18                      local -n file_ref=${file_v}_ref
19
20                      echo "$file -> $line: ${file_ref[line - 1]}"
21              done
22
23      }
24
25      a() {
26              false \
27                      foo \
28                      bar
29      }
30
31      trap 'bt' ERR
32
33      a

With a()'s `false` triggering the ERR trap I would expect output from bt()
to point at line 26, however, the BASH_LINENO holds, surprisingly to me,
line 27:

$ bash  ./t
./t -> 27:              foo \
./t -> 33: a

If I don't break the failing line then the BASH_LINENO is set just fine:

$ bash ./t
./t -> 26:      false foo bar
./t -> 31: a

Same thing happens if the ERR is triggered outside the function. So like
this:

1       #!/usr/bin/env bash
2
3       set -o errexit
4       set -o errtrace
5
6       bt() {
7               local line sub file file_v
8
9               for ((idx = 1; idx < ${#FUNCNAME[@]}; idx++)); do
10                      read -r line sub file < <(caller $((idx - 1)))
11                      file_v=${file//[\/\.]/_}
12
13                      [[ -f $file ]] || continue
14
15                      if [[ ! -v ${file_v}_ref ]]; then
16                              mapfile -t "${file_v}_ref" <"$file"
17                      fi
18                      local -n file_ref=${file_v}_ref
19
20                      echo "$file -> $line: ${file_ref[line - 1]}"
21              done
22
23      }
24
25      trap 'bt' ERR
26
27      false \
28              foo \
29              bar
30

gives this:

$ bash ./t
./t -> 28:      foo \

Not sure if this is expected so any hints would be appreciated. :)

Regards,
Michal