This is an uncommon situation, and not blocking anything, but it seems that there is no limit on the number of times you can nest stages in a Pipeline, even though as per the following, the limit should be two levels of stage nesting when in a sequential stage:

https://jenkins.io/doc/book/pipeline/syntax/#sequential-stages
"The stages within stages in a stage cannot contain further parallel or stages themselves, but they do allow use of all other functionality of a stage, including agent, tools, `when, etc."

Attached is a testcase:

Jenkinsfile

When this is run, it passes without any warnings or errors, even though this should be technically unsupported. Here is the log:
log

I'm unsure if we should warn users in this situation, or update the sequential stage documentation at https://jenkins.io/doc/book/pipeline/syntax/#sequential-stages