Keep in mind spam is spam! There is never a rhyme or reason to receive it.
I own many domain names and have received emails to random address on my domain even though they don’t exists.(admin gets all emails)
Also, like yourself I have received emails even though I only registered that address one place.
Neither of these conclusively point to a database breach. Not all spam gets filtered is the key. Spam can come from many sources a few I can think of off the top of my head are:
-Just random blast every address on a domain
-email data breach
-data in web browser cookies
-local computer hacked
-many many more…
With so many sources, it is difficult to say where something came from.
This of course is my opinion and I have no factual data to back it. But I can say this. I have over 20 email addresses forwarded to one gmail account that I have held for 15yrs. Some of these addresses can be found on public websites. That said I have less than 10 spam messages slip through gmail spam filters on a monthly basis.
Keep in mind I get 10x that in my actual spam folder.