Although not unprecedented, 5 times since 1914 (including 2018 as of election night), in mid term elections, the party of the President has gained senate seats and lost house seats. Gaining senate seats is usually a sign of a President with high approval ratings, (Kennedy 60%, Nixon 57%, Reagan low 40's, Trump mid 40's) whereas losing seats in both the senate and the house is considered the norm for the party of a sitting President.
“This election marks the largest Senate gains for a president’s party in a first midterm election since at least President Kennedy’s in 1962,” Trump said. … “This vigorous campaigning [I did] stopped the blue wave. History will really see what a good job we did in the final few weeks.”
But as you can see from the table, the party of the sitting President usually loses both senate and house seats. You would expect losses in the senate to equate to losses in the house and vice versa. After all, congressional elections occur both at the same time and in the same electoral environment.
This mid-term election markes the second largest senate gain to house loss in history (as of election night +3 -26) next to Wilson (+3 -61), then Reagan (+1 -26), Nixon (+2 -12) and Kennedy (+4 -4).
http://www.centerforpolitics.org/
http://www.voteview.org
https://fivethirtyeight.com/features/how-crazy-is-it-that-the-senate-and-house-might-move-in-opposite-directions-this-year/
https://www.studentnewsdaily.com/daily-news-article/trump-third-president-in-100-years-to-gain-senate-seats-in-midterm-election-while-losing-house-seats/