The other day I've been talking with someone on social media, and I said to them (explaining why I had been radio silent the previous day):
Ciao! Scusami, ho avuto troppo da fare al lavoro ieri
They corrected me and told me that I would need to write
Ciao! Scusami ma ho avuto troppo da fare al lavoro ieri
Now, both in English and in my native Russian, that would be a weird way of putting it.
You would only say "sorry but" when you are being accused of something or making excuses.
If you said "I'm sorry but I need to use the bathroom" when leaving the table, it would assume you're not normally allowed to use the bathroom; and something like "Hello! I'm sorry but I overslept" would be borderline rude, because oversleeping is not a valid excuse for being late.
In English, both these phrases sound funny and they would work better without "but".
However, I've noticed that Italian speakers do use this "sorry but" all the time when speaking English or Russian, so maybe Italian works differently.
Do you need to use ma after scusami, even if you're not making excuses?