Foreign Key constraints on xocolatl/periods

Hi,
I'm sending this to pgsql-hackers because Vik Fearing (xocolatl), the reviewer of
https://commitfest.postgresql.org/30/2316also has a repository with a pgsql implementation of said functionalities:
https://github.com/xocolatl/periods.

I have stumbled upon a probable issue (https://github.com/xocolatl/periods/issues/27), can anyone take a look and
confirmif the current behavior is the expected?  

Thanks!