Multiline command substitution - syntax errors with mysterious `+1`

I'm trying to break a long command substitution on to multiple lines, as discussed in [this answer](https://unix.stackexchange.com/a/82183/128023) . In a plain command pipeline, both explicit (\) and implicit line continuation work fine:

$ echo 'blah foo bar' \
> | grep -F 'blah'
blah foo bar

$ echo 'blah foo bar' |
> grep -F 'blah'
blah foo bar

Inside of a command substitution, they both fail, and slightly differently:

$ foo=$(echo 'blah foo bar' \
> | grep -F 'blah')
-bash: 4131
| grep -F 'blah') +1 : syntax error: invalid arithmetic operator (error token is "'blah') +1 ")

$ foo=$(echo 'blah foo bar' |
> grep -F 'blah')
-bash: 4133
grep -F 'blah') +1 : syntax error in expression (error token is "grep -F 'blah') +1 ")

I'm on bash 5.2.21 and .37. I'm testing at the command prompt, but my real use case is this in .bashrc:

SSH_AUTH_SOCK="$(\ls -l /tmp/ssh-*/agent.* 2> /dev/null | grep -F "$USER" | head -1 | awk '{print $NF}')"

What's happening? Where did the +1 come from? And is there a way to do this correctly? (I'm interested in portable solutions or bash-specific.) ### Edit: More context I'm using [Oh My Posh] and [Atuin] with [bash-preexec], all of which do something at prompt time. My example actually looks like this: They're set up in .bashrc like so:

eval "$(oh-my-posh init bash --config "${omp_config}")"                                                                                                                                                          # Get bash history number:
set_poshcontext () { export _myhistcmd=$(( $(fc -l -1 | cut -f1) +1 )); }
#                                                 Note the +1! ^^

[ -r "${HOMEBREW_PREFIX}/etc/profile.d/bash-preexec.sh" ] && . "${HOMEBREW_PREFIX}/etc/profile.d/bash-preexec.sh"
eval "$(atuin init bash --disable-up-arrow)"

To @Stéphane Chazelas questions:

$ typeset -p PS{1..4}
declare -- PS1="\$(_omp_get_primary)"
declare -- PS2="\$(_omp_get_secondary)"
-bash: typeset: PS3: not found
declare -- PS4="+ "

$ echo "$PROMPT_COMMAND"
__bp_precmd_invoke_cmd
_omp_hook
__bp_interactive_mode

$ trap
trap -- '__bp_preexec_invoke_exec "$_"' DEBUG