The problem only occurs when both underground belts have the same direction.
see also:
https://www.youtube.com/watch?v=nzscF_2BHDM&t=673s
[kovarex] [1.1.12] No warning when placing blueprint of underground belt on top of underground belt entity
Re: [kovarex] [1.1.12] No warning when placing blueprint of underground belt on top of underground belt entity
Thanks for the report, it is now fixed for the next release.