the reason for this is that in this case , "however" is being used as a conjunctive adverb or a adverbial conjunct i.e however is an adverb in this case acting as a conjunction.
As these two are independent clauses( a group of words that contain a subject and verb and delivers a complete thought) written before and after "however". so , however is used to separate two independent clauses with a semicolon before it and a comma after it .
IN SHORT, WE USE A SEMICOLON AND A COMMA WITH "HOWEVER" TO INTRODUCE A NEW INDEPENDENT CLAUSE IN A SENTENCE.
