When I was in school, we were taught to use a semicolon before the words however, therefore, although (don’t know what kind of word they are) when used in the middle of a sentence, and then they are followed by a comma. There are other words of this type but my brain cannot recall what they are.
Examples
I went to the store; however, it was already closed and I could not buy the bread.
The store was already closed; therefore, no one could buy any bread.
The store was already closed; although, the sign said it was supposed to be open.
I have found in my line of work that a lot of people are trying to shorten things up everywhere (like text language as an example) so I see a lot more use of just a comma before these terms and nothing after. That is not classic grammar.
By the way, there are cases where a word such as however is not with a semicolon in the middle of a sentence because it is not an independent clause before or after.