_comp_delimited loses completion on ambiguous choices

[scop]

Describe the bug

To reproduce

Make sure show-all-if-ambigous is off for readline. Seems this doesn't happen with it on.

#532 (comment)

Expected behavior

Versions (please complete the following information)

• Operating system name/distribution and version: Ubuntu 20.04
• bash version, echo "$BASH_VERSION": 5.0.17(1)-release
• bash-completion version, (IFS=.; echo "${BASH_COMPLETION_VERSINFO[*]}"): current master

Additional context

Debug trace