I don't have much experience with Bulgaria, but I can tell you this - they will eventually investigate it if there will be a reason for it. No doubt about that.
They can't really investigate much in terms of UBO's and this whole EU directive is absolute lunacy if you ask me because here is a practical situation:
Persons A, B and C are (practically) the UBOs of about 6 companies.
The documented legal owners of these companies are persons D (the car valet guy), E (a friend of a friend of person A) and E (an unofficial employee of A, B and C) which on paper have nothing to do with persons A, B and C.
The document link between these 6 companies is the fact that they share the same office (which may be practically true as well).
However here's a fun fact, there are about 3 more companies registered at this office which simply happen to not be based there anymore and the office secretary is kind enough to forward their mail to their actual office.
Now here's the even funnier fact: the first 6 companies share the same accountant who actually works in the office and that accountant happens to have power of attorney of all 6 companies.
That accountant also performs all bank transfers, transactions and cash withdrawals.
Any deals involving property, signing contracts etc are performed by either the legal owners of the companies themselves, an attorney with a power of attorney or again - the accountant.
Any company income is distributed as dividends to the legal owners D, E and F which they themselves withdraw in cash and hand over to persons A, B and C or simply if...say a car is being bought, it gets bought by the companies and the companies happen to provide persons A, B and C with power of attorney over the car.
Then persons A, B and C either pay for stuff in cash, deposit average amounts over time in their bank accounts or simply go to the casino, deposit as much as they wish playing roulette and at maximum, lose about 2-3% of it (sometimes even win some lol).
So please now that you have a complete picture, please tell me if you have to "prove" who the UBO of say...company 3 is, how the actual f**k would you go about doing so?
What exactly will you end up with? Arresting the accountant for being the undisclosed UBO? lol.
Arresting everybody in the office and asking them ARE YOU THE UBO!?
P.P. Wanna end money laundering? Revise and unify accounting requirements of the following: Banks, Night clubs, Sports teams and leave stuff like people trying to buy some Bitcoin alone for f**k's sake, ooh and simply ban cash payments and boom, you're done.
Wanna end secrecy of ownership? Well too fucking bad, you can't. Leave people the f**k alone with the EU bureaucracy.
Wanna end tax evasions? Go US style on citizens and prosecute overseas income by demanding information from foreign governments by simply demonstrating your big military dick.
Wanna actually run a country decently and let people prosper there? Do nothing of the above and counter THE REASONS people actually resort to doing the above and not the action itself.
Just my 2c as an EU citizen.