Basement "Live mode" glitch and inaccessible.

Product: The Sims 4
Platform:PC
Which language are you playing the game in? English
How often does the bug occur? Every time (100%)
What is your current game version number? 1.61.15.1020
What expansions, game packs, and stuff packs do you have installed? The Sims 4 Digital Deluxe, The Sims 4: Seasons, The Sims 4: Outdoor retreat, The Sims 4: Tiny Living
Steps: How can we find the bug ourselves? I cannot say, the bug that has occured can ONLY be recovered by recovering my saved game back to the very beginning, before I even moved my family into a lot. From the moment I moved a family into a lot and EVERY SAVE THEREAFTER IN THAT VERY SAME SAVED GAME, have the bug, where I can make and access basements in "Build mode", but cannot access the basement in "Live Mode", see video for reference.
What happens when the bug occurs? I cannot access the basement I build in "Build mode" when I go into "Live mode", if build under grass, the outlining of the room can be seen, but only if forcing going down to basement level by pgDn key.
What do you expect to see? I hope it can be fixed in some manner, otherwise I am gonna weep over all the lost progress of the save I've made, maybe keep on playing on that save, knowing full well I'll never be able to make basements.
Have you installed any customization with the game, e.g. Custom Content or Mods? Never used.
Did this issue appear after a specific patch or change you made to your system? Neutral/Not Sure 

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Dear Sims community and Sims support,

I am experiencing a devastating bug, that is making it impossible for me to make USABLE basements in my main save in Sims 4. See video here for more detail, because even a 1.8 mb video file is "too large" for this platform to accept.

I can make a basement and rooms for the basement in "Build mode", but as soon as I go into "Live mode" I cannot go down into the basement; There is a weird glitch where the ground floor seems to "get in the way" of me 1) Accessing the lower floor and 2) Even not going to the basement "overlay" mode, with the void around it. The only thing that happens is a visual glitch in the grass, as seen in the above linked video, outlining the underground room that is there, but is inaccessible during "live mode". And this happened just after I moved the very first family into a lot in the save, even before I had started BUILDING ANYTHING.

Also before you suggested that I should empty my cache, I have emptied both the Sims 4 cache files as well as the Origin Application cache files. And still the bug prevails in this, my main save. 

If This cannot be fixed it will be forever marked in my save history; I Will rename the save "No basement save :'(", and then have another save where I can build AND ACCESS basements and in time maybe even move my sims over there through the gallery; Alas with a fresh start, all prior progress and hours of play lost but not forgotten. 

I am getting both frustrated and desperate, so I am looking forward to any help, that this community can give me; Thank you very much in advance!
Sincery,
Klode_me