Transactions opened at the time the change of tier is started, they will be rolled back. Logins connected to the database will be disconnected. Contained database users will stay connected. Make sure your application is getting connected with a contained database user.
Query plans may vary after scaling up/down because the number of threads available for query changes, and the data cache and query cache will be cleared. This means when queries are executed the first time after scaling up or down, they may show poor performance and some timeouts may occur until data is uploaded to memory again.
When the database is facing high DTU usage, you may also face connection timeouts.