Why can't I cut out a circle from a ceiling through an existing center of a stair case?

Hi Sorry to ask maybe such a stupid question
Some may recall my endeavor with my huge game project
I am now have it UE5.4
but noticed some mistakes so back to SU
I forgot to cut a face where I sunk a spiral staircase through
I have been on this for hours now and just can’t do it
Feeling sooo stupid and frustrated come so far over the years and still get stuck with such basic failiures,
So I can cut out a circle IF not using the exact center from the center of the
support column.
The face is a totally separate group by itself
When I use the exact center it will create a new face
that I can delete but the old one will remain so no point
with the off center I said ok I try try to cut away with extra curves like a caterpillar munching the edge of a leaf
. But no way the spaces defined will NOT delete
Naturally I could cheat as this is just one spot in 18 levels on 50x50x50m model
BUT i want to understand WHY this happens and how to get around it
I had a similar issue with my bottom lake water
and then there was an issue of some spec of lines hiding within the intended circle
Normally this ceiling is navy blue so I changed the colour to make sure there is not one spec of anything within the intended circle

Thank you so much for any help

PS KUDS to THIS FORUM
I feel the need to mention compared to “UE forum” here is heaven!
Always an angel willing to share knowledge and offer answers and helpful solutions.
At Unreal Engine help is nearly existent or the occasional smirk like “go watch more tutorials etc.” I guess Open Source and the many youngsters playing around on or with it make the difference.
The main issue there even more prevalent than here is to explain a question using the right terms etc.
I joined a class for over 200$ and still fell that even that teacher couldn’t care less as long as one paid his fee his deed seem to be done even on the exclusive
FB group.
So thank you dear Forum members here!

For anyone on the same SKU model > UE road, tried many times with the fbx but always had missing parts.
Then discovered that tutorial that suggested to pass via Twinmotion which I did.
My HUGE model that now is near 6GB displayed PERFECT and then migrated with DATASMITH to UE to also display perfect .
I am not learning collisions etc wish me luck.
Still haven’t found out how to CUT out LAND around the initial game space floor around my model but I guess the day of Eureka will come as always.